Well, you asked for an opinion, so I will give you mine. It may not be correct, but it is what I suppose to be the case.
I think you are right that the UGAG game opened his eyes to where we are compared to where they are right now (and Clem's Son to some degree as well) - it may have confirmed what he was already thinking earlier this year.
Over the past 4 years, where he has experienced (and we experienced) 2 very poor non-bowl years, the frequency of what I consider unfair criticisms of PJ were being heard by him, and the naysayers were increasing, but they were still the minority by a long shot. (by the way, it is o.k. if you or anyone else believes some of these criticisms were fair - that is just our opinions anyway)
So, I believe some of this was building
I think he saw that it was very unlikely that he could recruit well enough to the system he wants to play (too old and stubborn to change probably) and compete at the highest level anymore
All of this kind of added up to him saying "do I really want the headache anymore?" when he believed he was not going to get where he wanted to get the program, and he came to the answer, "no it is not". I do believe some of the unfair criticism got under his crawl, but the main reason for him leaving, in my opinion, is because I believe he thinks he got this program as far as he could take it, with the current environment and the current restrictions/limitations at the Institute.
Again, just my amateur opinion.