I believe that Ammonium Nitrate requires a fairly high temp to detonate or a severe shock (e.g. blasting cap), but until it reaches that temp it will burn rather energetically.
I doubt that there was fuel oil mixed with it, but who knows what else was stored nerby and could have been unintentionally? mixed into some of the bags of pelletized Ammonium Nitrate.
Once I saw reports that repairs had been carried out on the door that day, I viewed the most likely scenario as careless worker improperly disposing of a cigarette or not properly shielding some procedure that threw sparks starting a fire, and no one who knew how to put out the fire able to extinguish it before detonation temp was reached. Ammonium Nitrate is both fuel and oxidizer for the fire, thus very hard to extinguish.
I see. So it likely burned heartily for a while then came the explosion of the remaining stock. The men may not have known the danger, so to run away.