Heres some numbers for those wondering: Tanks get 200 threat each when warriors and trenchcutters first spawn each time. So the first group always comes to tanks since there isnt anything else going on. As you get into group 2 and 3 though, they are spawning while healers are pumping out pretty decent hps, so thats when you see them breaking off for healers after a few seconds. And just to clarify if people dont know, healing threat is [(total healing done*.5)/number of active targets]. Talent multipliers can alter the .5 depending on how many points they have in it.
havent had a problem with groups 2 or 3 of that phase in the fight. The mobs are dead before the next group spawns so no massive heals being tossed around and the mobs run straight to our tanks everytime.