Something had to hit the primer when it fell with enough force to discharge the shell. I'm betting something was in the floor that it landed on.
Since the shell was not enclosed in a guns chamber the powders force was not concentrated in any one direction, and it had much less power. Likely the shot in the shell had very little speed and damage or injuries would have been slight. In fact the much lighter plastic shell moved with much greater force than the shot. It is unlikely anyone would have suffered serious injuries from a shell discharging outside of a gun. I still wouldn't have wanted to be in the area.
The guy in the video didn't seem to be doing anything irrespnsible or dangerous. A shell simply rolled off the edge of the table while he was talking. Most all of us have had similar things happen, he was just very unlucky in the way it landed.