Here is what I think happen, tank pull as usual, by going straight to boss tomahawk/shield lob and go pass the boss so that now the boss will face the wall. Healer and BLM for some unknown reason decide to reposition themselves right behind the tank, putting them in the direction of boss aoe. Due to how the camera works in this game there are times you might not see what's behind you or might not notice that the healer and blm have reposition behind you. I say might mostly because you would not expect healer and dps would go behind you after you have properly turn the boss.

So if the healer and blm for no good reason intentionally putting themselves in harms way becomes the tank fault? Sure the tank could reposition when he notice this, but give me a good reason why the blm and healer would move behind the tank in the first place? In this fight there is none, there are nothing to avoid once you are behind the boss.