It is, however, heat sensitive. At 32.2 degrees Celsius (89.96 degrees Fahrenheit), ammonium nitrate changes its atomic structure, which in turn changes its chemical properties.
When large quantities of ammonium nitrate are stored in one place, heat is generated. If the amount is sufficiently vast, it can cause the chemical to ignite. Once a temperature of 170 C is reached, ammonium nitrate starts breaking down, emitting nitrous oxide, better known as laughing gas. Any sudden ignition causes ammonium nitrate to decompose directly into water, nitrogen and oxygen, which explains the enormous explosive power of the salt.
"I am assuming that there was a small explosion that instigated the reaction of the ammonium nitrate -- whether that small explosion was an accident or something on purpose I haven't heard yet."
In fact, many countries in the European Union require that calcium carbonate to be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.