Isn’t that why one’s finger should not be on the trigger before the target is acquired? Even for SS??
// Hickey reaches for the AR-15 under the seat, releases the safety and begins to lift the gun. The second shot is fired by Oswald, hitting the president and Texas Governor John Connally. The president’s car and the follow-up car containing Hickey suddenly speed up. This is attested to by Secret Service agent Clint Hill. Agent Hickey, who is unstable because he is standing on the cushion of the seat, rather than the floor of the car, begins to fall back due to the acceleration of the vehicle, pulling the trigger of the AR-15. The gun is pointed toward Kennedy at that instant, and the bullet strikes him squarely in the back of the head. //
[Conjecture; movement of his finger onto trigger may also have been due to sudden movement of car & his loss of balance.]