Ummm, try pretty much all of the Zandalari, Amani, Drakkari, Gurubashi, and Farakki troll tribes? They have pretty much been aggressive antagonists since their beginning in Warcraft lore. Or maybe the Blood Troll tribe that is coming in the next expansion, which runs on a harsh, primitive matriarchy centered around gore, blood magic, and sacrificing their weak males to their ancient god G'huun?
No idea where you've been getting the impression that trolls aren't vicious or aggressive. Tribes you may deem as "friendly" (i.e. Darkspear, Shatterspear, etc) are actually in the minority lorewise. Every other tribe in the empire is usually xenophobic and/or war-mongering. It makes me wonder what reasoning Blizz will come up with for the Zandalari to side with the Horde despite having so many enemies in the faction, like the High/Blood Elves. Not even mentioning the troll wars, how will they have gotten over both Alliance and Horde beating them down at every turn, from original ZG/ZA, to Cata's updates, to everything we did to them on Isle of Thunder?