I just saw a new video of the doctor in question, before the removal on the airplane.
The guy had it coming, the police were quite patient with him actually.
Maybe it was one of those 'loss of face' situations?
High ego individual vs police, not a good combo.
http://http://www.dailymail.co.uk/news/article-4405764/New-video-shows-United-passenger-s-argument-police.html