That boss, from time to time, aggro other members of the party and the tank can't do anything about it, no one can, and after a short while, the boss will return to the tank if he/she is the one with most hate.

Few things you can do besides keep running, maybe using spring to get away faster, is casting Stoneskin and/or the water spell to push the boss (don't know if that will work but you can try it).