Your getting close.... My theory is that some unscrupulous, or perhaps ignorant, reseller had some mil-surp SLAP rounds that were somewhat corroded, and he tumbled or vibratory finished them to polish them up for re-sale. Tumbleing or vibratory finishing can and will pulverize the gunpowder within a loaded cartridge causing it to burn much faster / detonate.
Never tumble / vibratory finish loaded ammo.
Thank you for that thought. I would have never considered doing that with finished round but such would certainly explain a fast detonation. It goes beyond what one would typically call a hot round.
My theory would have been that the cartridge length was over spec and pinched the bullet at the throat. That would cause overpressure. But in watching the video, the cartridge went in easy enough, so unless his screwing on the cap drive it to wedge at the throat, your theory seems more plausible.
Drove...not drive....
My spell checker is arguing with me and winning even when it’s wrong...