It was a while since I read After the fall, but it was the senior partners that sent L.A. to hell. So a few minutes after the alley fight begins they're all cast into hell. I think it was as a punishment, them sending L.A. to hell. They sent Angel there, and made him human, to be able to control him, since they would need him for the Apocalypse.
Then I recall them tricking Gunn, who had been turned into a vampire, to think he could save L.A. if he built a shrine if bodies, but the Senior Partners used it to heal Angel instead? I'm a bit fuzzy on that part.
Any way, I've forgotten a few things, but in the end, Angel realizes that if he dies, the senior partners will turn back time/restore L.A. since they need him, and he kills himself(or tried to), and the Senior Partners turn back time, to the fight in the alley.