Be warned, ladies and gentlemen: There are a couple of minor spoilers for Wrath of the Lich King in this week's Ask a Lore Nerd. I don't think they're very substantial ones personally, but the choice is yours whether you want to continue reading or not.
What tribe, if any, did the dire trolls come from and if not a tribe, where in general?
Just how dire trolls come to be is largely an unknown, but they don't seem to be an independent race. They're just bigger, stronger versions of a troll. All of the tribes seem to have dire trolls.
There are a couple of quests in Wrath of the Lich King that might suggest dire trolls are regular trolls who have been 'empowered' but to avoid spoiling too much, what goes on in those quests is pretty different from seeing Jin'rokh the Breaker hanging out with the Zandalari. So all that we really know is there is no racial distinction between Dire Trolls and Regular Trolls of the same tribe. One is just much, much bigger and stronger for some reason.