I've read somewhere (can't find the link now) that the subject aircrew was from an associate airline (United Express--different operator than United who also operates regional jets for American and others, so not even technically their own employee. Other plane would not have been stuck on the tarmac. The crew had to get to Louisville that night to meet crew rest requirements. So that flight could have been cancelled/delayed hours in advance if no crew was available to fly it. The owners of that crew had other methods available to get that crew to Louisville, UA did not have to accommodate them on a fully booked flight, and in fact could not legally do so. I personally think there must have been some hanky-panky going on between members of the two flight crews. I can't think of any other reason UA would bend over backwards to fuck this up so badly.