We almost never know in these situations the reason, and the airline doesn't tell us. In this case it was revealed because of the public nature of this story.
I think if he would have calmly responded and explained his situation, then a different outcome may have been possible. He didn't need to be hysterical.
If I understand correctly, he was quiet but firm when he issued his first no and offered that he was a doctor who had patients waiting for him the next day.
Makes one wish that there had been a medical emergency on the flight and that they kicked off the only doctor on board.