That's just your vengeance at work, since Blizzard nerfed vengeance from multiple mobs the Garrosh tank will have much more vengeance than the add tank. If you grab aggro on all the adds the warrior tank should just taunt Garrosh.
He probably only managed to keep the first wave because he popped some cooldowns (for example bloodbath and dragons roar, two 1-minute cooldowns).