During the pre patch to Battle for Azeroth there was a lot of complaints and discussions about inconsistencies in the story presented to Horde and the story presented to Alliance.
An example being as a Horde player during the Ashenvale quest chains you show up and assassinate guards, are told to ignore citizens and actually heavily questioned/told off if you do kill one, and in further quests even help alliance citizens escape the small village in front of teldrassil, whilst killing the guards. From a war perspective, it's very much an honorable way of approaching the situation. That said, from the Alliance perspective, you show up after the Horde has passed through, and it looks like we burnt the whole place down and massacred innocents, not just guards.
I recall reading this starting discussions about why the inconsistencies were so obvious and it almost appeared as if Blizzard rush wrote the story with little thought. One discussion that always gauged my interest however was that there was some level of influence over the Horde and Alliance driving us to war. Perhaps an old god influence. This influence was causing each faction to see the actions of the opposing faction differently, or potentially causing these actions themselves and making it look like the opposing faction. In the above mentioned example a theory is that the Alliance have been influenced or deceived to think that the Horde have burnt down the village and killed innocents, when they actually haven't. Whether that's through deception or this influence doing that exact thing, is unknown. It was always an interesting concept to me, a 3rd party making the Horde look worse to the Alliance and vice versa.
Recently in Crucible of Storms we saw a mechanic on both bosses that immediately caught my attention. Now, looking at the Dungeon Journal for 8.2 Eternal Palace raid, the mechanic is also there. This mechanic is the 'Mind Control' mechanic, however it's been done slightly differently.
On both Cabal and Uu'nat, mechanics that 'Mind Control' you, actually just make you hostile to friendly players for a period of time, but you're still in full control of your character. This exact mechanic exists in the Eternal Palace raid, on the Za'qul encounter there is a period of time where all players are hostile to one another.
Now to me, this sounds a bit familiar. To attempt to point out the obvious we're seeing several mechanics that make FRIENDLY characters APPEAR TO BE ENEMIES but IN CONTROL OF THEIR CHARACTER. What if this, like theorised before is what has been happening leading up to Battle for Azeroth and during Battle for Azeroth.
This hostile mind control has been inflicted on the Horde and Alliance and whilst each factions actions aren't necessarily the most diplomatic, they're certainly not as bad as the opposing faction sees them as (albeit there are exceptions *cough* teldrassil *cough*). As a result the opposing faction sees each other as more of an enemy than we have in the past.