Uh, I'm pretty sure USC already has some recruiting violations to worry about. Kiffin is a smart guy. I think his bravado(and violations) in Tennessee was designed to put himself and TN back on the map. My guess is he tones it down a bit with USC's current issues. I also think he knew that winning in the SEC is a lot tougher than winning in the PAC10 at USC.
Because he made a big show about being in TN. I think, more than anything, TN is a respected program that was falling behind. A lot of coaches would kill their mother to coach at TN. Kiffin used it as a stepping stone. FWIW, when the first few coaches turned down USC, I kept thinking "I wonder if Kiffin is on their list?" He doesn't have a very good track record as a head coach but he's very much like a younger Carroll with his exuberance.