Reply #22: I think it's just stress.

[Marie26]

He seems to do it when he's nervous or anxious - which is almost all the time anymore. In Woodward's book, Bush said something about his jaw getting locked w/the stress before the Iraq War. I'll try to find the quote.

ETA: "There was a lot of stress," the President said. "Yeah, I felt stressed." All of the holiday parties at the White House added to the difficulties at the end of 2002. "My jaw muscles got so tight. And it was not just because I was smiling and shaking so many hands There was a lot of tension that holiday season."