What if I told you that Garrosh Hellscream is an honorable orc; a good leader commited to the Horde, uncorrupted by dark forces. What happened to the forthright mag'har Garrosh of Burning Crusade? What happened to that true hearted orc of an orc that Thrall put in power? What if his very worst moments in recent lore have been exagerrated, and have had critical details omitted, excised, or covered up? On top of that, what if his more benign acts have been overlooked, covered up, and downplayed? What if there was an agitator, a malignant influence working from the shadows to harm the Warchief's reputation, to turn the horde itself against him. What if the conflict between Garrosh and Voljin was engineered? If someone 'leaked' incriminating false intel that predisposed one another against each other. What if Garrosh was receiving false reports of seditious activities, and his seemingly egregious measures with the Kor'kron are not unfounded. What if agent provocateurs posing as Kor'kron commit acts of violence and brutality to conjure antipathy towards Garrosh?
Who would do such a thing? Who could do such a thing? Wrathion. Wrathion, whose motives have long been bathed in shadows, has this entire time been angling to make himself warchief of the Horde- to inherit one of the two greatest armies on Azeroth for himself. By conducting a campaign to sabotage the stability and PR of an otherwise loyal, competent, and successful warchief he has pushed the Horde to the brink of revolution. All the while he has been ingratiating himself to Alliance and Horde heroes, and earning the trust and friendship of Anduin Wrynn. At the final hour he will step in, 'help' free both the Horde and the Alliance from the 'threat' of Hellscream, earning hero status for his direct and bold involvement storming Orgrimmar.
How does this make him a cantidate for Warchief, though? There are many other more likely cantidates. Wrathion has garnered some esteem amongst Horde heroes, but his real angle has been in befriending Anduin Wrynn. What relevance does that have to the accession of the Horde Warchief, you ask? The Occupation of Orgrimmar.
Wrathion manipulated it to be that the Horde resistance thoughtlessly sought and accepted the Alliance's help to invade Orgrimmar and take down Garrosh (for lack of strength and numbers to do it themselves). After Garrosh was deposed, the Horde resistance found themselves in an awkward position; the Varian Wrynn's Alliance forces remained in occupation. Why wouldn't they? For decades they have struggled against the Horde as their primary rival, suffered great losses, and lived in fear. Now, at last, they hold Orgrimmar. Why leave and let the Horde re-establish themselves?
No, Varian Wrynn has insisted that Northwatch remain as a 'peacekeeping force' in Orgrimmar, to help rebuild and maintain security. In truth he never intends to let the Horde regain independence ever again, and Jaina Proudmoore stands proudly by him in this decision. Durotar and the Barrens become a neutral territories, with Northwatch guards patrolling Ratchet, Crossroads, Razor Hill, Sen'jin village, and Orgrimmar. Dissenters are 'disappeared' by squads of Kirin Tor 'peacemancers'. The outer territories of the Horde remain hostile to the Alliance, but with Orgrimmar under occupation, and each under threat from a unified Alliance, they are uncoordinated and unable to organise.
Of course, the subjects of the Horde are growing increasingly hostile at this Alliance occupation of Durotar and the Barrens. The resistance, led by Vol'jin, has turned from fighting the Kor'kron to engaging Northwatch in guerilla warfare. Taverns in Ratchet full of Northwatch troopers explode violently, Northwatch ships are rigged to sink at sea, lone patrolmen are sniped from the shadows. The situation is becoming untenable. Varian is unwilling to listen to reason, to risk allowing the Horde to reform and become the threat it once was, so he clamps down ever tighter than ever upon the Horde Resistance. This only provokes ever greater sympathy for the resistance.
All the while Wrathion, is becoming a leading figure amongst the Horde, he has withdrawn to Dustwallow Marsh, where he rallies a force he calls 'the True Horde'. Decrying the 'cowardly' tactics of the Horde Resistance, he calls upon the heroes of the Horde to stand by him with Strength and Honour. Little bombs will not make a free people, he argues.
As the one person who might be able to sway the King's mind, Anduin Wrynn approaches his father. He raises the hypotethical of ending the occupation without risking the Alliance to future Horde aggression. What if an 'acceptable' cantidate were found to lead the Horde. Not an Orc, not Vol'jin, or a fel tainted blood elf. What about a hero of the horde, who is also a trusted friend of the Prince of Stormwind himself? Peace must be given a chance, Anduin pleads. Varian refuses, the risk is to great to put on the word of a dragon (the son of Deathwing no less).
Having amassed a great enough force, Wrathion marches upon the Barrens with his army. Overwhelming the forces therein, he honorably gives them the opportunity to withdraw to Ratchet, bearing with them an ultimatum: in two weeks either Varian Wrynn meets Wrathion to discuss the terms by which the occupation will end and peace shall exist between the Alliance and the Horde, or Ratchet shall fall. Varian refuses, and Wrathion makes a feint attack against Razor Hill- scrambling Ratchet defenses Northward, Wrathion's main force emerges from Ratchet bay and Ratchet falls as promised. Ratchet is not sacked, and prisoners of war are treated humanely. As a Gesture of goodwill Wrathion extends that all prisoners will be released unharmed if Varian but meets him to discuss a treaty.
At Anduin's insistence, Varian accepts. He is impressed by the discipline and restraint of the Horde forces under Wrathion- Wrathion explains that the Horde has a noble heart, just one that is easily cast astray by tyrants. Wrathion explains that for years now it has been his greatest wish to bring peace to Azeroth, but that without the extermination of the Horde or the Alliance, that could never be possible. Such it was that he has set himself to gaining the position he now occupies- one where he might negotiate peace on behalf of the horde. Wanting to waste no time haggling, Wrathion proposes a generous deal: The Alliance is to ccease it's occupation of Durotar and the Barrens; Ashenvale and Hillsbrad are to be returned to the Alliance; A permanent peace treaty is to be made, wherein the Horde swears never to make a first strike against the Alliance. Despite the generous deal, Varian asks for more; that Vol'jin and Sylvannas be turned over to the Kirin Tor for imprisonment in the Violet hold for War Crimes. Wration sympathises, but insists that were he to comply, the Horde itself would fragment, and the treaty would be for naught. Varian persists, and at last Wrathion makes the compromise, Sylvanas and Sylvanas only shall be handed over- but Vol'jin and his Horde Resistance shall remain unpunished. Giving great pause, and after much urging, Varian accepts.
Varian keeps his word and pulls occupying troops out of Durotar as power is transitioned to Wrathion. A grand day of ceremony takes place; Wrathion is sworn in as Warchief, A peace Treaty with Varian Wrynn to be signed immediately after, and elsewhere, (an alarmingly compliant) Sylvanas is taken into custody. At last the Alliance shall know peace, a tear of joy forms in Anduin Wrynn's eye- it is finally happening. Wrathion is sworn in, and the treaty is signed- the Warchief and High King shake hands and a great applause is underscored with fireworks.
-then, teleporting into the fray is a near-dead Jaina and several bedraggled Kirin Tor mages. Jaina to call out, but one of the retching mages that teleported in with her convulses and turns into a ghoul, leaping at her. Anduin dashes over and obliterates the undead mage in a flash of holy light. Struggling to maintain consciousness, Jaina rasps to the young Prince "A trap... scourge blight, the Horde bombed Dalaran with Scourge Blight. Sylvanas is free..." Varian at last notices Jaina and Anduin amidst the revelry, and the look of horror on Anduin's face. Before he can act he is immolated in black dragon flames, as Wrathion assumes his (now juvenile rather than whelp) dragon form. Anduin's howls of despair are drowned out by Wrathion's cackling, "You fool! Did you think you could tame the Horde? Ha! By handing it to me, you have made it more dangerous than ever." Turning to the crowds of Orgrimmar, Wrathion continues "Warriors of the Horde, you have been shamed and dominated by these Alliance Weaklings. Claim your honor back, may the valleys of Orgrimmar flow with their blood!"
Jaina manages to teleport herself and Anduin out of Orgrimmar with her last mote of strength, and Anduin is changed somewhat- his youthful naivety scarred by Wrathion's betray; the seed of vengefulness planted in his heart. His father died for his unrealistic notions. Varian was right all along, he was the High King the Alliance needed; there could be no peace with the Horde. Together with Jaina he would see Wrathion and the Horde pay for their treachery. In this case, the only way to peace is violence.
From here you have the much more popular Anduin as High King ruling closely with Jaina in a 'ballsier' alliance. The Horde is led by a goddamn Dragon, a Black Dragon, the last of his kind. The Warchief and the High King have a very personal relationship- once friends turned enemies most bitter.
TL;DR All of Cata and MoP has actually been Wrathion's gambit to become Warchief. Garrosh wasn't even evil, Wrathion betrays Anduin and kills Varian. End result; Horde gets Dragon Warchief, Alliance get badass Anduin.