He is just pissed Slash is more respected than he ever will be. Its the typical Ego vs Ego fight that plagues nearly every band ever made. Its just that this is so much more high profile than the five guys in the garage next door.
The band doesnt even have to be successful for this shit to occurr. You have to be pretty batty and talented to play in any kind of meaningful group in any case and chances are good that the lead guitarist and the frontman will headbutt with their egos now and again unless they are one and the same. I say this out of personal experience