I see in Larry's explanation the pressure changes related to the powder not igniting when it should by being held back. Instead a bullet moves out increasing air space and everyone here knows to not download a slow powder so the results are the same as a drastic download. A drastic download can cause trouble even before the bullet leaves the brass. You still have a blockage.
But Larry is spot on, freebore or throat erosion, long seated bullet with no tension, strong primer action.
I figured out the why with just one event but my rifle suffered no damage. If I can sit at the bench and figure it out it sure is not rocket science.
Looking at all the stuck bullets we had with the .454 using under max loads, powder packed behind a bullet stuck in the bore that failed to light at all because of the stupid SR primer, it is a wonder none have blown up. What would happen if the powder decided to light? Would the gap save the gun? The LP mag primer cures the problem in the .454.
Thinking back to experiments with the .44 mag, I determined my poor accuracy was due to different bullet movements caused by the primer. It was then I also discovered case tension had to be even from one piece of brass to the next, again case tension that crimp did not cure. Discarding the magnum primer back around 1980 with 296 powder things improved and bullet stutter went away.
NEVER download H110/296 is said here over and over but why would you do the same thing by moving bullets?
Theories are all over the place but your bullet becomes a bore obstruction, plain and simple.
Bullet jackets do not weld to the bore either but remember when the high pressure revolvers came out and the wrong bullets were used? The core shot out and left the jacket in the bore. They fixed it with magnum bullets.