I got an add, as far as theorizing what happened. Of course, this would not be the root cause, but contributory.
Before retirement, when I was over Safety, Health, and Environmental at a plant, Our Fire Brigade had a leader who was obeessive with learning and leading Emergency Response operations. On the surface, hearing him, you'd think he was a large city Fire Chief.
But when that event happened, that confidence and leadership didn't materialize , and part of that is that no matter how much you drill for an event, you never how they will respond. Actual emergencies are rare, and it is impossible how people will respond.
I am guessing that someone or a few in USSS team at Trump's rally had that same kind of moment. Like choke in a sports analogy.