Sometimes when a college program has sustained success under a particular coach, that coach will want to jump to an NFL job. This is particularly true if that coach has NFL experience of any kind. Because of the way the NFL coaching cycle works, losing a coach to the NFL will happen way late in the college coaching cycle. That means that unless the program is promoting from within, it's going to be stuck trying to get someone on short notice. Continuity with the successful coach's regime will always be a high preference, not only because that coach was successful but also to try to save that year's recruiting class.
So sometimes a program will make a Ron Zook hire. He was an assistant on the successful coach's staff who hasn't really done a lot to prove himself on his own, but he checks the continuity box. The program thinks that maybe he can keep things going because he was there during the glory days. He won't, but hopefully he at least stocks the program with good recruits while he's there.
USC made its Ron Zook hire when it went and got Lane Kiffin to replace Pete Carroll. Kiffin had not done anything with the Raiders or at Tennessee to prove he'd keep up Carroll's level of success, but he at least had been on the staff. National Signing Day was looming—USC hired Kiffin on January 12—and so under time duress, he became the guy. Kiffin did win ten games and recruited well, but he also got fired on the tarmac at LAX.
Given plenty of time to make a new hire after Kiffin's Week 5 firing, Pat Haden made another Ron Zook hire. Steve Sarkisian did a good job of digging Washington out from the wreckage of the Ty Willingham era, but he had four consecutive 5-4 conference records before the Trojans picked him. He hadn't proven himself to be worthy of one of the top jobs in the sport. But because he had a tie to the Pete Carroll era, Haden went with him.
After last night's brutal 17-12 loss to Washington, it's not looking good for Sark. He isn't taking the program back up to the top level, and after the debacle at the Salute to Troy in the preseason, it's not out of the question that USC pulls the plug on him if the team finishes 6-6 or worse (which is possible with a 3-2 record and games against Notre Dame, Utah, Cal, Oregon, and UCLA to go).
Making one Ron Zook hire is understandable. No one wants the good times to end, and finding someone to follow a legend can be tricky. But you don't make a Ron Zook hire twice, and it seems that's what USC did.