I think he was just killing time while the director and cinematographer were setting up the camera for the scene.
The earliest reports were that there was no film of the incident because the crews were still setting up the cameras for the angles required for the scene.
Baldwin may have been fidgety due to the delay, and started practicing his draw movements while the crew were still moving about. They may have even needed him to be in position for proper blocking of the scene in the camera's viewport. While he was standing there, the propmaster should have kept possession of the gun, only handing it to Baldwin when the director was ready for action.
-PJ
Dang! Thanks for the reply.