Here's the brief nontechnical version, as I understand it. The powder charge in the case is ignited from the base. Thus unburned powder it sent into the neck of the case as the bullet exits.
When there is too much powder for the small bore get out of the case quickly pressure skyrockets until there is sufficient volume for the powder to begin flowing.
It's just like pouring 60 grains of 4350 in a powder funnel made for the 17 REM. Except without an ignited primer behind it. It just isn't going to go through that little hole in a reasonable fashion.
After 30+ years I never experienced it either. But I blew some primers and described what was going on to Kirby who diagnosed it as powder bridging.
ball powders flow smother thus no bridging.
I've been working on a frontal ignition where the powder is ignited at the base of the bullet. Its a lot of work but bridging isn't a problem and recoil is reduced due to not sending that weight of powder down the bore.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!