Being ready for it, Optimus grabbing the gun merely had it sliding out of Cliffjumper's ready grasp, not yanked from it. The problem with the sudden firefight was that Cliffjumper almost sat up straight, attempting to get a good look, systems powering up--

Before the minibot himself throttled everything and forced himself to curl up as much as possible, turning off shrieking alarms about fight-or-flight, as the readiness was just eating up what little reserved he did have.

If he actually needed to act, he'd need as much energy left as possible, and he wouldn't have that if he used it all up on fighting with himself about wanting to do something and staying down.