To my knowledge, the explosives which become unstable are the nitroglycerin based explosives. The explosive materiel will actually "sweat" the nitro out as droplets. If you can get a drop on your finger and the temperature is right, you can flick it on the ground and it will give a nice snap or small "explosion". Have that happen while still on some propellant, you have a nice ammo "cook-off".
When the battleship (I forget the name) gun turret went up while they were loading propellant, I figured they had an old lot which had sweated and was unstable. After the homosexual suicide thing blew up in the Navy's face, they relooked it and concluded "early powder misfire". Almost guaranteed to occur with large temperature swings which was the case for the Navy 16" ammo as their storage bunkers which was storing the WW2 propellant weren't sate of the art. BTW, the Navy hadn't had any of that ammo or propellant made since WW2 so you figure what condition it must have been in sitting in some damp moldy ammo bunker.
If you want to see something cool, check out my recent W33 update on wikipedia. If you want me to answer questions about it, I will as part of a new thread.
Good Hunting!
Tom