Originally Posted by SoundgardenRocks

Billy has actually fired Jimmy twice. In the original lineup, midtour for Mellon Collie, Jimmy and the pumpkins keyboard player(I think he was the keyboard player) were doing heroin. When the keyboard player died, and Billy found out about Jimmy's drug problem, he fired Jimmy. After Machina(2000), the Smashing Pumpkins disbanded.

Later, in 2005, Billy announced a reunion.(But without James and D'arcy) After their reunion album Zeitgeist was released, it was announced Jimmy left the band. Jimmy said the split was amicable. But later on Corgan said he actually fired Jimmy. When asked why, Corgan said Jimmy was still a "destructive person."

I guess nobody really knows how or why they split except for Corgan.