Probably a gradual heat buildup by something else burning in a closed room and slowly increasing the room temperature to enough of an extreme. Enough cartridges popping at once could make a funny roar in a nearby room instead of distinct pops. Ammunition storage should be ventilated at least a little to avoid tight containment of pressure.
Avoid storing that stuff in a space shared with a flame (water heater, lights, electrical appliances without circuit covers, etc.). For outbuildings (e.g., shop), build a tough steel cabinet with steel doors that are *not* airtight for small amounts of ammunition and/or handloading supplies (capped top vent preferred). Build another one for paints, solvents, etc. Isolate far from welding rigs, heaters and the like.
Take it from military practices. Any who store very much ammunition (common for some competitors or anyone who like to fire unusually large amounts of ammunition) would be better off building a safer, ventilated (e.g., capped pipe on top), concrete box for it outdoors and in the ground. Ammunition can be sealed in airtight containers that won’t contain combustion too much (plastics, very thin metals, etc.). Same for anyone who stores much fuel. I don’t need to store much of either (not a match competitor for now and don’t use much fuel around the place) but would build safer, separate, outdoor facilities for it, if I did.
Ammunition won’t “blow up” much, if it’s not tightly and heavily sealed, although large quantities of it burning can cause burns, scare fire-persons, etc. (heh, heh...”firepersons” instead of “firefighters”).
Excellent post, thank you.