The video is pretty jumpy, but it appears the rear rivet failed. It just needs to be re-riveted. I have a Lancaster built Polish underfolder that works like a champ. It is not clear what caused the rivet failure. 10,000 continuous rounds? Shok Buffer? (these are a very bad idea) Bad rivet?. Did the gun come that way from Lancaster. Was it abused? The video does not tell me much. I have seen Colt AR's fail (ammo problem, not Colt's fault). I have Ruger P-85's simply drop their slides (I have no idea why). A firearm is a mechanical device that can fail, wear out or be built badly.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball