Why did Chris Petersen step down?
I would attribute it to job burnout as an umbrella title for a host of things. His passion was gone. His heart just didn't seem to be in it his last year. I've heard that he is disappointed in the direction that cfb is headed and he just wanted to do something different. Family is important to him and he is not willing to sacrifice it any more to his career. Plus, financially, he's set for life.
I think he just decided that he didn't want to do it anymore. He certainly wasn't forced out.