from what I read on it, it stems from an injury he suffered in high school.
I tore my ACL back in high school. There is scar tissue in there from the surgery and my knee cap sometimes gets caught on it and dislocated. I'll spend the next month or two with a bum knee.
I think with stafford it's a lot less of a problem, supposedly he dislocated his knee in high school and once you do it you are more susceptible to do it again.
EDIT: Cracker I love your sig but honestly if i am reading talkbears at work I have to make sure no one is around...