 When Trump drops down and is pounced on by bodyguards, he's now partially hidden by a large black speaker box between the shooter's view of Trump. So Crooks tries to shift his position to the right, so he rolls to the right to see if he can see around the box enough to get another shot off at Trump. And that puts him finally in the view of the far sniper team, which kills him. The first sniper team visible in the form of it, he can't even see him