A dirty trick though without much precendence or motive. Why would a gate agent decide they want to cook something up to save United 4000 dollars? Why would the gate agent decide to create a situation that doesnt exist to deny boarding, when they absolutely dont need a reason to do so ffor an overbooked flight. Also we do not know that the denied passengers were not just treated as IDB. Angry passengers upset about delays are in airports everyday and most of us have been upset or raised our voice sometime in our life in an airport and the reaction is never to deny boarding (at least I haven't heard or seen such).