All members are at fault. Nobody bothered to adapt.

Your group's main problem was the AOE.

Situation #1 : DPS didn't bother to position themselves behind the boss, as a tank, you could turn the boss around.
Situation #2 : Tank didn't bother to turn boss around, as a dps, just position yourself behind the boss.

Simple enough. Competent players know how to adapt to various situations. Your issue was very manageable.