When adds spawn during a boss fight and auto-attack the healer, I think it has something to do with aggro tables on a chain. When i'm tanking sometimes i notice that aggro has been given to the healer before the adds have even been fully spawned and clickable/tabbable. Like with Psychflare in AK, right as he is visible the red arrow is auto-targeted the healer even if i have no ST/Regen or any healing buffs. I don't believe your pulling aggro off the tank, its more like the games algorithm is just forcing aggro on you. As a tank i anticipate this and time all my flashes accordingly.