I don't think anyone involved expected or dreamt this would happen. In retrospect there are any number of ways this could have been avoided but, to the people there, this eventuality wasn't remotely anticipated. Passengers boarded, CC trying to get everything ready, then a request to find X seats. Seems they found X-1 without issue. Final one, chosen by algorithm, pushes back. Argues, won't budge. So CC walk back annoyed that their rightful request has been ignored.Think how can I make this guy realise I am within my rights and he must stand, walk and go. OK, I'll find someone in uniform with 'security' on their vest to make the point. That will convince him. Never think the security guys anticipate they will only be called upon when muscle power is needed and with that mindset they board the aircraft.
Everything is still OK until they lay hands on a 69 year old professional. That's when the jobs are lost, careers terminated and the millions leave United's bank account.