Like I said earlier, while I know a crappy drummer or two, band's downfalls are related to egotistical people, at least in my experience.
Actually, speaking of drummers, I know one that drove every member of this band I was in nuts with his scheaming and stuff, so we all left, and the ones that stayed, well, lets just say they'll all be leaving sooner or later, so, I guess thats another reason bands bust. You can have one guy who wants to run the show, but has no clue how a real band works, so everyone else gets ticked off and leaves.