According to Murdoch:

Quote Originally Posted by Pragmatic View Post
If there is sufficient quantity of ammonium nitrate, it can generate enough heat to catch fire and keep the fire going, without the need for an external catalyst such as a flame.
According to Scientific American:

At high enough temperatures, however, ammonium nitrate can violently decompose on its own.
Ammonium nitrate decomposition can be set off if an explosion occurs where it’s stored, if there is an intense fire nearby.

What Is Ammonium Nitrate, the Chemical That Exploded in Beirut? - Scientific American