Warren Report on Agent Hickey at the time of the shooting. But why was Agent Ready called back? At that moment, the President's car was only traveling 11 mph.

From: https://spartacus-educational.com/JFKhickey.htm

Special Agent Ready, on the right front running board of the Presidential follow-up car, heard noises that sounded like firecrackers and ran toward the President's limousine. But he was immediately called back by Special Agent Emory K. Roberts, in charge of the follow-up car, who did not believe that he could reach the President's car at the speed it was then traveling. Special Agent George W. Hickey, Jr., in the rear seat of the Presidential follow-up car, picked up and cocked an automatic rifle as he heard the last shot. At this point the cars were speeding through the underpass and had; left the scene of the shooting, but Hickey kept the automatic weapon ready as the car raced to the hospital. Most of the other Secret Service agents in the motorcade had drawn their sidearms.