Seriously, who thinks the USSS would involve themselves in political theatrics?
That's kind of self-answering. DU-style Progs do.
What happened, AFAIK right now, is pretty simple. Someone wanting to make a scene brought a concealed sign into the rally front row area. Whe he reached into his clothes to get it, another attendee mistook the motion for going for a gun and called out, "Gun!". The USSS heard it and did their job, getting Trump offstage swiftly. No cowardice, just USSS agents doing their job and Trump having the sense to cooperate.
It would have played out much the same had it been a Hillary rally, except, maybe, for the word "swiftly".