I personally liked the scenario. It was goofy, and didn’t make sense, but hey. At least it wasn’t “We’ve never OFFICIALLY been invited. Maybe you should do that.” (Low blow.)
But in all seriousness, the Orcs were, in our timeline, destined to destroy their own world, as Miko pointed out. And that was with Blackhand as our leader. Blackhand, as far as I can see, wasn’t that bad of a leader. He kept things in line, was logical, and though he had pride he never wanted to be ‘da bestest ork in orkdom’.
Then we have Grom, who leads the Iron Horde. The same Grom who was kept his title by virtue of being feared, who drank the blood on the grounds of ‘Why not?’ (Weakness or something but originally he just wanted to be da bestest ork in orkdom)
My personal theory is this: many Draenei would not forget what the Iron Horde did to many of them, and may not see the distinction between Gul’dan’s orcs and Grom’s orcs. Not to mention, the Iron Horde is very militarized and ‘gunpowdered’. If Grom went to war with the Botani, as Vanndrel pointed out, then he, being the genius tactician he always was (the same guy who said “Hey, that’s a Demigod? Time to cleave-sheave-my-reeves”) probably allowed the use of flamethrowers in the Evergrowth or other places of Draenor. Those fires ran rampant, probably killed a lot more than was expected, and maybe a lot of innocents got hurt.
Not to mention the rebellions. If they’re having dissenters, all it takes is six orcs to dress up as Blackrock Orcs and attack the Draenei to put more pressure on both sides. Draenor is not a happy place to exist in, and we didn’t do it any favors by being there. Plus, in our timeline, the Draenei and Orcs were never ‘buddies’. They co-existed, mostly because the Orcs could never dream of sacking their cities and taking them over. They did not have the military might to do so, divided and unempowered by Mountain Dew.
Then again, the Draenei in this particular scenario as it was presented did have to deal with the Legion. Sure, we beat them back side by side, but the Naaru saw how easily the orcs could be swayed to conquest. They let destruction come to Draenor, and thus they could pose a threat to the rest of the universe. Whether that is right or wrong it is a decent enough point to start building resentment–on top of whatever else might have happened.
Edit: To add more to the side in favor of the Mag’har being relatively innocent in this conflict, since I noticed my post is pretty much ‘This is what the orcs could’ve done bad all the time’ (I am trying to be unbias in this theorycrafting) the Draenei could believe the orcs caused MORE destruction by fighting back. If the Orcs refused to be Lightbound, and fought back with all their destructive potential, than they did indeed help destroy Draenor further in an attempt to try and suvrvive.
Facetiousness aside, I am mostly rambling at this point. But to answer the original question: A lot of things, but no one can tell. Draenor is a savage world, and you give a race born from that savagery the capabilities to enhance their chances of survival on a destructive level.
Tl;dr: I have no idea, but two peoples who’ve lived side by side after committing atrocities against one another, on varying levels, are NOT going to be friends no matter what happens. And Draenei live a long, long time. They’re not going to forget this as easily as a finite race who replaces itself every 70-100 years.