This surprised me too. I know he's playing at an extremely high level and there's up and down action, but I think as a college point guard you should have the capability (when called upon) to play 20 minutes fairly easily. These guys are in the best shape they're ever going to be in and to get cramps after working for that amount of time doesn't make sense to me.
My personal opinion is that he does actually have the fitness level to do it, but didn't get enough water or potassium during the day. But I'm no dietitian...