Bassist Nate Mendel (also of the Foo Fighters) was apparently instrumental to the reunion:
I was a bit of a pain in the ass to Jeremy and William. I had tried before, but thought it wouldn't harm our relationship too badly if I called and tried one more time. Next thing we know, we were playing our songs again and had a major tour booked. The biggest deciding factor for me in a reunion was the fact that it would be all original members.
Check out the interview (and retrospective) here.