Those morons were shooting surplus 50 cal SLAP rounds. It's a smaller diameter projectile engaced in a plastic sleeve callked a sabot. They're designed for velocities higher than a chrome 50 cal barrel can withstand and are made for armor penetration. Shooting in a single shot rifle with a muzzle brake is a REALLY BAD idea as the brake peels the sabot back every once in a while. The rifle they were shooting looks like an Armalite AR50 and they use a "tank brake" on them, as does the Barrett. They're notorious for posing problems with the SLAP. It sounds like the sabot peeled back and became a little plastic boomerang.
A 50 isn't going to travel downrange and then bounce back in a perfectly straight line and then strike the person firing. You have a better chance of winning the lottery every week for the rest of your life. 50's don't ricochet. They can deflect, but typically only by a few degrees. Most often, they just go through whatever they strike. If it's too thick, the projectile is literally vaporized on the strike. The standard 50 round is a 700 grain projectile travelling 3000 FPS. That yields 14,000 Ft/Lbs of energy. That's about 9 times that of the round fired by an M4/M16.