Bob Stoops has two sons who are both seniors in HS and play football. Perhaps he wants to get away from the grind and be able to enjoy his sons senior years. In addition to his $2 million house he just built around Norman, he also has a house in the Chicago area. If Brian Kelly has another bad year and Notre Dame fires him, which seems pretty likely, I could see ND making a serious run at Stoops. Chicago isn't that far from South Bend, and Stoops would be a great hire for ND.
This situation with Bob Stoops is very similar to Urban Meyer a few years ago, when Meyer took off a year for health reasons. His name was mentioned for every big-time CFB opening that came up. I could see this same thing happen with Stoops. Is there more to the story with BS stepping down? Maybe. Maybe not. If there is something that caused him to "retire", it will eventually come out. It will be very interesting to follow what happens with Stoops this season and how many feelers/offers he's going to get, which will be a lot.