First, college coaches leave all the time, they are either fired or leave for another job. For UConn football, not in a P4 conference, if a coach is successful he will leave. Isn't that better than firing him for failing?
Mora did a great job at UConn, but he fumbled the exit. It didn't help that the last game was in Florida before the Thanksgiving break so players, in general, went home and were not in Storrs. Thus, Mora wasn't able to tell the players in person because nobody was on campus.
Mora has seen the pushback on his departure and he is trying to fix the narrative surrounding his departure. If you read Millen's view of how things went down, Mora made up his mind that he was leaving weeks ago. Millen said Mora and he started texting about the job 2 weeks ago. Heck, Mora's wife visited Ft. Collins to check it out. I would trust what Mora said to Millen, a long time friend, over what D'Amelio thinks might have motivated him to leave.
Finally, there were many reports that Mora was interested in Stanford (been available all season) and Cal and he was identified as a candidate. He wanted one more shot at a P4, but it wasn't going to happen at 65 years old.