Others have said that his religious beliefs explain his actions. Both arguments would be valid if this man was sitting in section 202 behind the basket. However, this man was sitting on the court, dressed to play, and receiving a large sum of money to do so. What Abdul-Rauf did was not religious. In fact it was just plain insulting.
People stand for the national anthem for many different reasons.
Mostly, they stand out of respect, a respect as individual as the person standing. Abdul-Rauf's failure to stand was disrespectful to the fans who had paid, and paid well, to see him play. Being from New York, I have attended many hockey games where the visiting team was from Canada. At all of those games, the anthems from both countries were played. All of the players and most, if not all, of the fans stood for both songs.