Rumor has it he was having a bit too much fun in Lincoln. Seemed to be trying to re-live his college years. Got thrown out of some bars, was sometimes late to practice, missed some recruiting visits due to social life. I've heard more specifics, but all is secondhand info, so I'm not going to put it out there. In fact, all of the above is secondhand info, but there's a lot of buzz about it in the Nebraska community.
My thought is that he's a good coach - the return "home" to a "dream job" was just too much, and he buckled under that pressure. But agree that if he becomes a candidate, references should be checked, and care should be taken (as with all OC candidates).