I thought that too. But from what I've read, once the cop topped the ladder, the shooter pointed the rifle at him. The cop then fell down the ladder during his retreat. After that, the shooter immediately fired at Trump.
If I peeked my head over the roof and found a rifle pointed at me, I’d duck too. Silly not to, and if he fell....
Who knows if that’s how it happened ?
It might relate to our snipers hesitancy in firing as Crooks attention and gun was distracted from the podium for a time.
All speculation....
One thing that needs no speculation is that the SS needs a house cleaning just like the rest of deep state.