I'm betting that cartridge had its bullet seated way too far out and it jammed solidly into the leade when he tried to chamber it. Most likely cause: inadvertently short stroking the press handle. The fact that he couldn't subsequently eject the cartridge says that bullet fit in those cartridges was/is pretty darn tight, so crimping would've been superfluous (and would be in future with the same lot of brass/bullets). Normally in a situation like that the bullet stays put- jammed into the leade- and the case pulls away from it when ejection is attempted, unless like now it was inordinately tight in the case neck.
Any cartridges left over from that batch? If so, maybe it's time to start pulling bullets to see if I'm all wet with this conjecture. Mic the neck wall thicknesses and inside diameter of necks (maybe after running a couple back through the sizing die to get a true pre-seating measurement), and compare to the bullets y'all were using. You only need a couple thousandths interference fit to secure a bullet in a case neck.
Compressed powder charge that could've pushed the bullet back out of the case too far? It happens. But then the case would've shucked itself off the bullet upon attempting ejection.
It would seem you guys experienced a perfect storm of small things that added up to a catastrophe. Just what each of those small things were remain a mystery.
