Theory/headcannon: The Sargerei

[Ethenil]

Imagine the draenei going from world to world: staying no longer than a few decades, having to leave because of the oncoming Legion. For twenty five thousand years. In these places, the draenei had few children, because they knew it wasn't safe and they were all in imminent risk.

Then, one day, they arrive on Draenor. They crash there, and find it good. Perhaps because of the fall of the Genedar the Legion could no longer sense them, and there they remained for 10 years, 20 ,30.. A hundred years passed and still they were safe.

They began having children. But these children had never had contact with the Legion, had never known the terror. So when they arrived in Force in Draenor, through the orcs, many faltered and sought the demons. This is why there are so many Sargerei on Draenor: it wouldn't make sense for an old draenei to despair only now, but it would make sense for a native of Draenor. Only a few old draenei, like Othar, because of some increasing grudge with the others also turned bad.

[Therougetitan]

This is actually kind of canon from one of the books in the Sargerei garrison campaign

http://wow.gamepedia.com/The_Eredar_Destiny

[Combatbutler]

Imagine the draenei going from world to world: staying no longer than a few decades, having to leave because of the oncoming Legion. For twenty five thousand years. In these places, the draenei had few children, because they knew it wasn't safe and they were all in imminent risk.

Then, one day, they arrive on Draenor. They crash there, and find it good. Perhaps because of the fall of the Genedar the Legion could no longer sense them, and there they remained for 10 years, 20 ,30.. A hundred years passed and still they were safe.

They began having children. But these children had never had contact with the Legion, had never known the terror. So when they arrived in Force in Draenor, through the orcs, many faltered and sought the demons. This is why there are so many Sargerei on Draenor: it wouldn't make sense for an old draenei to despair only now, but it would make sense for a native of Draenor. Only a few old draenei, like Othar, because of some increasing grudge with the others also turned bad.

There are several manuscripts which more or less swayed the young ones, they were pretty much just power hungry, like many of their kin before.

http://wow.gamepedia.com/The_Purpose_of_Unity

We draenei are the chosen race... civilized, cultivated, keenly intellectual.
There is no technology that our minds cannot grasp, and no magic that our arcanists cannot master.
Our civilization on Argus was magnificent to behold.
Our people were never slaves nor cowards, hiding from their potential.
Our people have the means to conquer the stars!
Unite with me, brothers and sisters, and let us take what is rightfully ours!

http://wow.gamepedia.com/The_Eredar_Destiny
Thousands of year ago, the demons of the Burning Legion came to our homeworld and offered our leaders a great gift.
Two of them, Kil'jaeden and Archimonde, saw the wisdom in their offering, but the last leader, Prophet Velen, refused.
He and his followers fled from that world, forever hunted by those who once offered their hands in friendship.
Velen was a FOOL! And we are descended from those fools, who would turn their backs on paradise!
No longer, my brethren! Today, we rise up and bow to the demon lords. They have given us another chance to share in their power.
This, my brothers and sisters, is our TRUE destiny. Today, we walk in the footsteps of our eredar forerunners, and claim the future that is rightfully ours!

http://wow.gamepedia.com/The_Promise_of_Eternity

The Legion's work must continue uninterrupted. There are those on Draenor who wish to disrupt their plans, and chief among them are the unbelievers - draenei who still follow the Light.
Destroy our kin, and keep our masters' work unhindered. Defeat the draenei and we prove our loyalty to the Legion. Crush Shattrath, and the demons promise us the glory of immortality!

[Ethenil]

The thing is that it isn't stated anywhere that this is the new generation that is turning evil - which doesn't make a lot of sense. Why did Othar decide to betray Velen only now?

[Combatbutler]

The thing is that it isn't stated anywhere that this is the new generation that is turning evil - which doesn't make a lot of sense. Why did Othar decide to betray Velen only now?

Well the text refer to those, who refused the gift as forerunners, indicating the target group of the sargerei are younger Draenei.