Glad there were no injuries
I had a cartridge go off in the open chamber of a 410 shotgun many years ago, when the chambered round failed to extract, and I rammed the next one in to the back of it.
I suffered only powder burns, and ringing ears for a few weeks. The cartridge did not blow the primer. Most of the force went out through the case walls.
Tests that have been run on purposely exploding cartridges in tube mags have had similar results, usually, the primer moves only slightly, or stays put, and the case rupture realeses the pressure required to properly ignite the powder.
If the case in the chamber ruptured, it's entirely possible that the gas was vented through the mag.
That, or something impacted that primer.
Can you do a test?
Put a dummy cartridge with a live primer in the mag, keep body well clear, and bang the stock down hard on the butt.
Check primer, for marks if it doesn't go bang.