Much of the time it's the singer that's the problem.
But then there's the occasion when you think it's the singer's problem then you realize the guitar player is a crackpot. For years I thought DLR's antics drove apart VH, then after the fiasco with Hagar and the falling out with Michael Anthony it's clear who has the problem in that band.
But you're right. Most of the time it seems to be the ego of the singer.