I think reality punched him in the gut.
I can't put my finger on it, but he did appear less energetic in the 2nd half of the season. Heading into the Bama game 7-0 with a very energetic fan base - it was the best he could possibly ask for in setting up for that "big season". Alabama firmly established that the premier teams in the conference are still head and shoulders above us. A&M then reinforced the message. I think it was a moment where he realized that winning championships here is tough - probably much tougher than he previously accepted in his own mind.
Now, I'm not knocking the program. I think we can have success at MSU and, when the stars align, be in a championship hunt. I'm just thinking Mullen had a medium term plan for what he could achieve at MSU in terms of Ws/Ls and that point in the season punched him in the gut and proved otherwise.
It will be interesting to see how Mullen reacts. I'm sure he's worked 10x harder than he ever has in the last few years and probably expected more wins because of it. Does he walk the talk he does with his climbing the mountain stories, amp up recruiting, address scheme issues, and do the best he can to get us to the next level? Or, does he accept the comfort zone he's in currently with 6-8 wins and decent bowls. I think most MSU fans have an appreciation of our historical performance and are happy with that. Mullen has (or should have, at least) bigger goals than that so there's a chance he could be throwing a small pity party for himself after reality sank in.