I've noticed this agro thing, and my best guess is one of my totems. What I did to compensate:

First group that enters the room (2 mobs), hammer of the righteous.
Second group (2 mobs), avengers shield.

I actually move out a little in front once the get to me and do this. It seems to work pretty well for me so far, but does require quite a bit of micro.

I didn't have much luck at all with the "group them in the back of the room" approach, but maybe I just don't understand what he's saying.