I think what happened there is that Curry, who was in the lead car, thought the shots came from the overpass. He ordered on the radio to get some men into the railroad yard. The police responded and ran there. When the people saw the police running into the railroad yard they ran up the grassy knoll to see what was happening.
Or maybe it had something to do with Lawson making a fuss about Holland, Dodd, and the other railroad workers being on the overpass.
SS SA Winston Lawson, 12/1/63 report:
We were just approaching a railroad overpass and I checked to see if a police officer was in position there and that no one was directly over our path. I noticed a police officer but also noticed a few persons on the bridge and made motions to have these persons removed from over our path.
SS SA Winston Lawson WC testimony:
Mr. STERN. What were the instructions that you asked be given to the police who were stationed on overpasses and railroad crossings?
Mr. LAWSON. They were requested to keep the people to the sides of the bridge or the overpass so that-or underpass-- so that people viewing from a vantage point like that would not be directly over the President's car so that they could either inadvertently knock something off or drop something on purpose or do some other kind of harm.
[...]
I noticed a few people along the right-hand side I can recall now, and more people on the right-hand side than out in the center strip median which is there, a grassy center strip. There weren't many people on the left at all. I recall thinking we are coming to an overpass now, so I glanced up to see if it was clear, the way most of them had been, the way all of them had been up until that time on the way downtown, and it was not. There was a small group, between 5 and 10 that looked like workmen. I got the impression, whether it was wrong or not I don't know, that they were railroad workers. They had that type of dress on. And I was looking for the officer who should have been there, had been requested to be there, and I noticed him just a little bit later, that he was there, and I made a kind of motion through the windshield trying to get his attention to move the people from over our path the way it should have been.