I didn't see what they were shooting at, but the ping sounded like a fixed metal plate (instead of one mounted on a swinging hinge). It would have had to be perfectly perpendicular to the X plane he was shooting on. What's really wild is that the return trajectory had the bullet on a path to hit the ground just in front of him and it still bounced up on the same X and Y axes and hit him instead of deflecting to a different path.
Moral of the story, always shoot with a trap if you can and if not, don't mount your target plate where it can't swing when hit, and shoot from behind a berm if at all possible.
I had a buddy a few years back that shot a .44 mag at a plate and it came back and tore the side of his neck. He ended up with stitches and a decent scar.
His dad bought him a hat to remember it by, it was a .44 mag with "do you feel lucky, punk?" written next to it.