Judging by the famous photo, the Secret Service was shielding him with their bodies and trying to drag him away from the scene. That would give them control over his lateral movement but wouldn’t prevent him from standing. The agent shielding him doesn’t reach above his shoulders, and she’d have a hard time stopping a “265 lb.” man from standing while only gripping his waist.
Judging by the famous photo, the Secret Service was shielding him with their bodies and trying to drag him away from the scene. That would give them control over his lateral movement but wouldn’t prevent him from standing. The agent shielding him doesn’t reach above his shoulders, and she’d have a hard time stopping a “265 lb.” man from standing while only gripping his waist.