One won't ever know, but the working assumption then is he didn't leave for NIL money or a gig at a bigger school. And you're not going to convince me he quit in the middle of the season hoping a bigger school whom he hadn't already spoken to wouldn't be spooked by that.
So I'd be looking at a personal, rather than professional, explanation. He wanted/needed to be closer to home, he had an issue with someone he was supposed to be working closely with, or he just wanted to go somewhere where he thought he could win more football games.