Isn't Oklahoma's AD widely regarded as one of the best and most stable in the business? I don't follow Okie that closely but hard to believe he and Stoops aren't basically joined at the hip.
Also, the Mixon episode got bad press nationally but is of a pattern with how Stoops operated there for a long time, presumably with the university's OK. He took in Dorial Green Beckham after Mizzou booted him for various drug and woman-beating episodes, and signed other recruits various programs - including Notre Dame - wouldn't touch. If Mixon was the last straw, that'd be a change of course.
And it's not like they haven't been winning.
So yeah, I'd suspect the guy's just tired of the grind and needs a break.