I still want a petition for tauren rogues :<
Presumably it was a trade.
Trolls taught the saner magic-wielding Orcs arcane magic, and the Orcs taught the less-sane magic-wielding trolls warlock magic.
Frankly works better than the "Trolls druids were around all the time. They were the tigers you were all killing."
Dammit, missed the moment... Lokann, ten posts ago you were at 6,666 posts. Post count of the Beast!
THe easy explanation is; Warlocks are "mages gone bad". Trolls have mages, hence they have warlocks.