It seems United Airlines overbooked their flight and needed to make room for four United employees who needed to get to work the next day in Louisville. They offered financial compensation of $400 and later $800. But when no one volunteered to get off the flight law enforcement got involved and here's the viral video of how this passenger was dragged off the plane.
According to other passenger on the flight, the man who dragged off was a doctor who needed to be at the hospital the following day, hence his not volunteering to give up his seat. The videos have gone viral and United is getting quite a negative response from Twitter users.