We don't know, but it was reported he stepped away so he and his wife could focus on their kids in their last years. He was originally supposed to be our DC when Sark was hired, but something fell through.
Also, he wasn't the primary DC, just the co-DC, so he didn't have the same high pressures that other coaches had. He wasn't leaving unless he got an offer he couldn't turn down (us) because of who we are in the name, and we are in his league now. If anyone doesn't think that will help, I don't know what to say. Guys like him with his passion and genius level defensive IQ don't just stop remembering what they're good at. He went back to Georgia because he, Smart, and Bobo were all roommates in college. It made sense, he got them two NC's in his first two years to get them over the hump.
I don't get why people just went from chastising PK in the game threads to now being upset that he's getting to go home and we get another elite DC. It's a win-win.