I don't think Herman had anything to do with it, and I'll be pretty surprised if Stoops coaches again. Being a college football coach at a major program is more work than ever. Some might remember that when Mack took over at Texas (and Stoops at OU), almost all of the recruiting happened for a given class in the two months before NSD. Now you generally have to kiss these kids' asses beginning at least two years before NSD if you want to have a shot at landing them. And as the NCAA has liberalized recruiting contact rules, the workload has only increased. It's a constant and year-round grind -- far more so than when Stoops was beginning his head coaching career.
When a person has dedicated almost all of his waking life to something for nearly two decades, and has made tens of millions of dollars in the process, it's not hard to see why he might just decide that he has had enough of the hassle and wants to enjoy life while he's still relatively young and healthy. And, in Stoops' particular case, even if he says otherwise, I imagine health has weighed increasingly on his mind. He has already outlived his father.
I keep seeing Sooners say that Herman lost to SDSU, for some reason. He wasn't at UH then.