Well, the Mixon thing was 3 years ago, so I seriously doubt it. He has two sons who play ball, and he has mentioned in the past that he wanted to watch them play if they made it to the next level. I think one is going to be playing for Northwestern, and Stoops recently purchased an estate in the Chicago area. I'm sure it's difficult "feeding the monster" for 18 years, so he may have decided to hang it up for a bit while his boys play. I wouldn't be surprised to see him come back to coaching again in some capacity. Supposedly he will remain with OU in some form, probably for consulting/recruiting stuff.
Some were speculating about a possible huge sanction of some sort coming down, but I think that would have leaked quite a while ago if that were the case. I think he was just waiting to make sure they got past NSD so that recruits wouldn't bail. I can't imagine Lincoln Riley agreeing to coach a program that was getting ready to have the NCAA come down on them.