I was under the impression that they blew up when they were killed, but I could be mistaken on that point. All my hard modes has been with guild members, many I have known through MMOs (and real life in some cases) for years. With everyone doing their best and min/maxing their rotations, the boss in question has died every time before damage from either the boss or the adds has been an issue. I realize that this isn't very helpful to someone who is struggling with the encounter. All I can offer is that I will talk to the others I have been doing hardmodes with and ask if they have been doing something during that fight that I just haven't been aware off.
Its definitely not when they die. They were blowing up for me with over 80% health. I was in there with some fresh 50's so it may have been a gear issue.
I'll go back in and see how it goes.
Is the boss supposed to be movable though?