Alternative theory: maybe the guy just didn't want to coach anymore.
He's had a nice run, won a lot of games, and made a lot of money. Maybe he just woke up one morning and said, "I don't need to do this for the rest of my life. I'm going to cash in my chips and enjoy life."
Sure, we can all cite cases of these incredibly driven coaches who sleep in their office and focus on FB 24/7. That doesn't mean that every FB coach is like that. If you're really wealthy, in your mid-50's, do you really want to spend the next 10 years begging 16- and 17-year-old kids to come and play for you? Not to mention dealing with boosters, NCAA regulations, compliance, and knowing that you can work your butt off and have it all go to hell if a couple of 18- or 19-year old kids do something stupid one weekend. (ask Mr. Claeys, if you can find him.)
Again, I could be dead wrong, but maybe Stoops just said "screw it" and decided to walk away while he was young enough to enjoy life.