Once it was found that it was demon blood which corrupted the minds of the orcs, perhaps stop treating them like worthless slaves and start building a better relationship with them, the orcs became extremely docile once the demon blood wore off, some even got extremely lazy.
The humans instead chose to stomp them down at every turn, vengeance, basically, racism.
It had a violent outcome as a result, even when the humans had Thrall to act as a medium between the two. An educated orc.
Prejudice and bigotry.
The Darkspear trolls joined the Horde after Thrall and the orcs saved them from Kul’tiran humans, and Naga.
The Goblins joined the Horde after the Alliance was determined to ‘leave no witnesses’ and proceeded to hunt down the goblins, they were saved by the Horde.
The Blood Elves joined the Horde because the Alliance was too busy spying and potentially destroy buildings essential to their survival, invading Quel’thalas too.
The orcs, tauren and forsaken are potentially the only races in the Horde currently that have no excuse for being hunted down by the Alliance. The tauren joined and associated with the orcs willingly, the forsaken are… well, forsaken. And yes, the orcs had ancestors that did bad, but the Alliance then also determined that the younger generation who were not a part of that bad deed, also did a bad deed simply for being orcs.
That is prejudice and bigotry.
Hell, even Garrosh, despite never having been a part of any of the events of the first and second war, as a matter of fact, he and his clan was locked away due to a disease, but he was charged with warcrimes committed during the first and second war.
Prejudice and bigotry.