Look at what happened to JFK. The guy's routinely going around in a motorcade waving at crowds, just like Eisenhower before him. At every event before Dallas, they're following strict security for the motorcades. That means, all buildings secured with no open windows, no driving below a certain speed (so no sharp corners), security on the running boards and motorcycle outriders surrounding the car, SS, local police and even military units all over the place.

Then one day they go to Dallas. Here, for some unknown reason, once they get to Dealey Plaza, every one of the above rules is violated. Windows in buildings are left open. The car makes a 135deg. turn onto Houston St., slowing down to 11 mph (I don't know what the minimum allowed speed was exactly but it was certainly higher than this. There's no SS on his running boards and the motorcycle outriders are reduced in number and kept behind the car (rather than next to it where they could serve as human meat shields). Local police told not to take part in security, and military units told to stand down. A few seconds later: Splat.

Point is there's precedence for the SOP of the SS to be compromised. To suggest it was an accident that ALL of these rules are broken in one time and place when they've been followed every other time is simply not plausible. The same applies to the Trump rally. There's no way you'll ever convince me the SS not doing their jobs that day was an accident