I might be wrong, but I believe one of the biggest flood migrations to be indeed MoP, Throne of Thunder to be exact. Primarily because there were a lot of "Beast" classified bosses, and trolls having both berserking and a 5% damage increase to beasts put them firmly at the top for deeps classes and most tanks (since we still had lulz-vengeace at this time) with most healers opting goblin for the utility.