As a younger man I had several bolt shotguns (because they were cheap) Many like the one shown in the video were sold by Sears, Western Auto, hardware stores etc. (you get the picture). The metal used in many of these guns was substandard. I can remember broken extractors, broken firing pins, and even one broken locking lug... on a 16 gauge that only had 2 lugs in the first place
My bet is that the gun had one or more damaged locking lugs. He was messing with the bolt trying to make sure it was locked. The bolt wasn't locked fully in place and it fired the bolt backwards into his face.
An interesting question... Why would you be shooting a ancient bolt shotgun on a video at an indoor range in the first place???????
Mark