I'm sure the fact that Tony Ball is a legend over there probably worked in his favor. Either way, Thomas will be a great addition to that staff and hopefully that will be the beginning of a long ascent in the coaching ranks. Couldn't happen for a nicer guy.
I also noticed last night (per Matt Barkley's twitters) that Tee Martin is joining Kiffin's staff at USC. While this sounds like a good move for Martin, you have to think about what dancing with the Devil will get you in the future. Probably an ass load of recruiting violations that ends badly with another NCAA tent-pitching outside of the athletic facility.
I would assume that Joker's tenuous position as HC was probably a determining factor here, as he could be gone next year, if they aren't "up to snuff".