I imagine United has the equivalent of an 'Airline Duty Manager' for that port. If so, they have not managed the on load of the crew and offload of the pax well.
If the paxing crew presenting at the gate was a surprise to the gate staff / duty manager then that is poor communication within the Airline. Either way staffing levels and poor training will be at the root of it. A calm, well trained, experienced person in authority at the gate would have prevented this escalating to the point where security was required.
Ensuring that staff are calm, well trained and experienced requires mature executive management decisions that value those traits.
Does executive management recognise and value these unquantifiable traits? If not, there is the root cause of the problem and the cost is now slightly more 'quantifiable '.