I had a similar occurrence many years ago - late 70's with my .458 2". The powder I was using was HiVel 2 with a standard primer, I think and was less than a case full, looking for .45/70 factory load equivalence for testing on bears.

The granuals of powder were yellowed and scorched, but no burn or not much at all. Later, full case loads with that powder worked just fine, so the powder was OK. I chocked it up to the less than a case full, with the straight sided case.

I had just been reading about John Buhmiller's dual primer system due to poor ignition in the large cases. Made sense to me and I stopped trying to find reduced loads with unknown powders, like HiVel 2. I don't know where I got that powder, but actually had 5 pounds of it (paper cans with pop-out tin tops) and the .458 2" rather liked it.

The bullet was pushed forward, but not completely out of the case, as it was barely started into the lands and extracted with the case, but was a bit too long to eject. I had to partially pull the bolt to get the round out. I was concerned another might burn a bit of powder and lodge the bullet into the bore, with the pressure blowing the bolt back when it was lifted. THAT happened to a friend of mine at a range down South in Vancouver. The edge of the spoon handled bolt sliced his palm to his wrist quite badly.

The only thing I can think of, was a bad primer in your instance as I expect the load was a normal one you'd used before. Mine happened on the first one attempted. I pulled the rest and didn't try to fire them.