I would need you to elaborate on what exactly "default aggro" is. Monsters tend to leash onto whoever has aggroed them until regen is casted and then they hunt the WHM to the edge of the atlas in a fountain of bottomless rage. The only other thing I could think of is when you enter a boss room and as soon as the tank pulls the boss it momentarily targets/scares the crap out of someone else in the party before jkin' and returning to the person who pulled.