Ahaaaa!!!
Gotcha now.....I'm pickin' up what you are puttin' down now....... 
That adds to the possibilities.......
In that case, heck, the pin itself (where it goes through the frame and barrel link could possibly be worn enough to let it slip....or the holes in the frame could be wallowed out....but I'm inclined to go with the former and say it is the pin diameter just making it enough of a sloppy fit to work itself out.
Sometimes thousandths will do it.
After jumping in late and tearing down my own full size S&W 1911,.....the detective "little voice" in my head saw the light and agree with the strong likelihood in agreement with Pegleg.
Something has become "wallowed out" right along those lines. The gun compensated under duress. Poor thing,....

Considering the tight tolerances of the S&W, if a wallowing were to occur at the barrel link, (a moving and flexible part subject to wear), and the pin that is so relied upon, is a very real possibility.
Compare to another. Great post. for those that cherish their 1911's...It's a tune up on a 57 Chevy. No special tools required, Just a credit to John Browning, who knew some parts will wear out with rampant shooters....

Between Wilson Combat and Para,....a solution will be at hand soon.
Again, good post. and a heads up for others.