It sounds like it was maybe primers that were cooking off; if there were “thousands of rounds” going off, the firefighters would have to have been more than 50 feet away, and there would have been significant damage to the surrounding structures.
It can be scary ....decades ago, Dad had a workbench fire that started when a soldering iron touched on a rag. Unfortunately, it was where the reloading supplies were stored too ... it was quite a mess. The smoke did more damage than anything ...and POP! POP! POP!
Rounds that explode outside of rifle barrels mostly just throw some shrapnel around from the shell casings from what I have seen.
I’m not so sure...
Didja see that episode of Myth Busters?
To all the hoarders out there though: Happy now?
No way they might crack down on it now. Right?
Nope. Cook off rounds (at least those that don't involve explosive oridnance) don't generate enough force to do much damage. Was a test done a number of years ago on energy from cook offs, and none were able to penetrate a fireman's call out coat.