OT turns the tank stance off because you don't want both the MT and OT eating cleaves by standing in front of it, but yet you don't want the boss flipflopping between two tanks fighting for aggro so that they can dump cleaves on the rest of the group, nor do you want the OT purposefully doing less DPS to make sure the boss stays on the MT.

If an OT is so far into his MAX DEEPS rotation to not notice the MT died, then that's simply on them and when you get back up, remind the OT that he's gotta pay attention. Simple enough.