My guess is that the gun is opening slightly when fired, and is pushing the auto safe on. Take a small lump of the oil-based modeling clay kids use, load and close the gun, place the ball of clay just to the right of the back of the top lever. Push it on smartly, but so it protrude above the bottom of the lever. Firing the gun will tell. If you have to pick up the clay from the ground, you have found the culprit. Now, what causes it to open on firing?
That's what I was hinting at in my first post. If the gun is well worn, with the lever to the left and the barrel flats contacting the watertable, they will "bounce" upon firing and the lever will open just enough to actuate the safety rod. Gerald says the gun is tight. But if it has had work done (metal added)on the hook and it was not properly mated to the bolt, there can be a wedge effect that causes the gun to start to open when fired.
Smoke the barrel extension (hook) slot, or color the inside of the hook with blue magic marker. After a few opening/closings, you should see that the bolt is biting only on the top (middle) third of the hook surface. If the bolt is contacting the first (bottom) third of the hook surface, it is not engaging properly.
If you find the lebver is moving per Dennis' suggestion, this could be why.