Either the case mouth or the case rim. You can actually see the case bounce off the wall and hit the box of ammo in the video. For the unseen shooter theory to work, the gun would have to fire at exactly the same time and in exactly the same place that the case hit. And then whoever took the video would have to post it with a misleading caption.so in theory a case mouth smacked a primer? I guess its possible, though the energy of a ricochet off a padded wall is unlikely to have the energy. It looks and probability wise, and common sense wise, that someone behind the guy was playing with a gun, and shot that ammo tray.
As unlikely as the empty setting off a live round is, it's far more improbable that someone accidentally fired a gun at exactly the right time and aimed exactly correctly so that it hit just as the empty case hit and in the same spot.