2NH₄NO₃ (s) → 2N₂ (g) + O₂ (g) + 4H₂O (g) Only happens spontaneously at high temperature/pressure. But of course being an explosive reaction, once it starts in

2NH₄NO₃ (s) → 2N₂ (g) + O₂ (g) + 4H₂O (g) Only happens spontaneously at high temperature/pressure. But of course being an explosive reaction, once it starts in a little pocket of material, all the neighboring material experiences a lot of heat and pressure, and blows up. And then the next layer, and the next layer, etc. If improperly stored, pockets form with different pH values that cause it to spontaneously occur at lower temperatures. Or a spark, or a compressive event in an unstable patch, etc.