Stoops went 8-5 and was feeling (a little) heat and got blown out in the bowl game. Oklahoma had a top 25 offense with Trevor Knight, but they had a really bad defense (like bottom 40). Who so happened to coordinate that defense?
Mike Stoops.
Bob didn't want to fire his brother (who just recently got fired at Arizona). He then used Heupel and Jay Norvell as the scapegoats instead of firing his brother. He then hired Riley running basically the same system, who got the job in short order as HC when a lot of people assumed Heupel was next in line. Stoops hired Riley to fix Knight, which Riley did not do and instead gave the offense to Baker Mayfield. Riley fired Mike Stoops after Bob retired.
That's why Heupel holds such a grudge.