I'll take that as secession.
I get the feeling that the Dunmer were being treated like crap before this conflict. So I'd say its Ulfric, and his peoples fault.
Based on what evidence or conjecture?
Also whats the excuse for him treating the other races badly as well?
Khajiit? Ignorance. Just about everyone thinks they're [censored] - even Imperials - because the majority of their renown comes from highwaymen. Compound this with that Khajiit were the second race to join the Aldmeri Dominion.
Argonians? They're mammals, but you can only tell because they have briasts. These things are extremely alien and alien things are hard to relate to. Furthermore they look like Dragons. Compound this with that Dunmer had enslaved them. I understand why the proud Nord people only see them as a beast race.
Imperials? Duh.
Bretons/Reachmen: They want Nord land and no matter how you slice it, it is nord land. Nords were the first humans down from Atmorra and they haven't really moved since.
Redguards: I haven't seen any negative opinion of redguards unless you count the mercenary warband and they were turned away because they were a warband - not because of their race.
Orcs: I haven't seen Orcs treated poorly either.
Bosmer: As a personal shortcoming, I have a hard time telling them apart from dunmer in Skyrim so I can't really comment on this one.
Altmer: Boy howdy, I wonder.