Some dungeons have mobs that spawn mid battle. Such as Arboretem when you are killing the walls with all the bees. Little bees keep spawning a lot and get instant aggro on the healer due to regens and stuff. Its a lot easier if the healer just runs to the tank so the mobs can get picked up by tank aoes instead of running around like crazy. Makes the most sense to run to the tank instead of having to chase them down to get it off the healer.