those ligaments/tendons. when the muscles get tired, they can't protect as much. it also affects your form, which can lead to injuries too.

think about a simple bench press. after you reach a certain point, you can barely lift the bar and your technique gets all fugged up and anything can happen.

with that said, you can't say it was only fatigue. shit happens. had this occurred in game 2 of the season, his minutes wouldn't be in question.