Baldwin said on 60 minutes or whatever show that was, that they were practicing drawing from the holster and aiming in the direction of the camera. Eventually, he pulled the hammer back, but claims he didn't pull the trigger.
My theory is that he grasped the trigger while removing it from the holster and sloppily kept the trigger pressed the entire time. The single action colt requires that you pull the hammer back manually. You can't just pull the trigger and shoot. However, it will immediately fire if you pull the hammer back and let go with the trigger already pressed. You see this used for rapid fire in some old westerns.
That is my theory anyway, but I haven't seen any real proof of this, or anyone else pitching this theory. But my theory jives with the fact that humans malfunction far more frequently than well engineered machines do.
H