I made an account just to reply in this thread. So grats u.
As a raider who tanks 10 mans as a DK and raids as an affliction lock I had to post to say that this is 100% absolutely on the tanks. And I say that as someone who has both died from the aggro reset on Marrowgar as the lock and *let* someone die as the tank.
Are the tanks running away from Marrowgar during bonestorm?
Are they getting pinned in by fires?
Because honestly they shouldn't be doing either. Bonestorm is pitiful damage especially against the tanks. I always make sure I've got IT and PS refreshed on Marrowgar during Bonestorm and try to always stay within melee range. And the fires? You can just run through them, only standing in them will cause you to take damage.
And to the guy that said DKs don't have the tools to pick up Marrowgar quickly, I really couldn't disagree more. I think we've probably got the best tools of any tank.
But it's just downright silly to tell a lock that he can't dps during a phase of a boss fight because the tank can't watch DBM and be ready to taunt. Or, he's not a 'team player', like someone said. I mean, whether he dps'es or not, if the tank isn't in position because of incompetence someone's gonna get aggro just from proximity whether they're dps'ing or not.
Still, in the future you shouldn't even be near Marrowgar when he comes out. If he's got like 6 seconds or left on bonestorm and he's not near to you, I try to move towards him since he'll likely do one final big move that will get him away from you.