Could the fire in the AN have heated enough of it rapidly enough to yield sufficient oxygen to create an explosive atmosphere that then went bang and produced the required supersonic shock mentioned in a subsequent post to set the rest of it off?I can still remember AN from school chemistry lessons, it is a very effective oxidiser. When a very small amount was heated in a test tube over a Bunsen burner it just rapidly decomposed. The resulting oxygen caused a smouldering match to re-ignite almost explosively.
As to welding, there was a disastrous fire in Düsseldorf Airport in the 90s caused by gash welding procedures.
The station armoury in Wildenrath burned down in September 1990, also due to gash welding procedures.
Oxygen is often regarded as being the key to life but if you "soak" an environment in it can become explosive when ignited apparently. Breath it when subjected to an external pressure of 3bar or so and it becomes toxic, if I remember rightly.