Even if it wasn't a passenger overbooking, it was overbooking in that they failed to properly account for the likelihood of having to ship crews around their network. If their occupancy ratio was lower, then there wouldn't have been a problem, they could have fit the crew in. If they reserved 4 seats for crew on every flight, again there wouldn't have been a problem. If they had even reserved 2 seats and run sufficie…
Many (some? most? warning, weasel words) airports operate at capacity so just adding more flights might not have been possible even if they wanted to
See, for example, the fights going on at Love Field over gate space