Bill McCartney, the Colorado coach from 1982-1994, suddenly (and shockingly) resigned after an incredibly successful 1994 season where his team wound up in the top five. When he was asked what you call it when a guy at the top of his profession quits with so many good years left, McCartney said, "Un-American."
However, he had home issues (his daughter, who infamously got knocked up by CU's late quarterback Sal Aunese, had been knocked up by a second one of his players and his wife and he were having some serious relationship issues due to an affair he'd had in the 1970s), he was an outspoken conservative Christian coach in a very liberal community, and he himself said (and I think of this with Stoops) "Not everyone has a job as stressful as being a college football coach" as he spoke of all the missed meals with family, missed vacations, and time out on the road recruiting.
So I hope Stoops doesn't have any hidden health issues, and I hope he enjoys his next adventure.