Well ventilated, fresh, farm grade ammonium nitrate is way less of a fire risk. Poorly stored, confined aging ammonium nitrate stored with who knows what contaminants is a very different beast. Assuming the AN was pure and stable when placed in the warehouse (not even a sound assumption), the passage of time allows it to degrade allowing for the build up of various nitrogen gasses and who knows what could leach out of the warehouse itself. There is also the real possibility that some or all of a shipment of seized AN was already mixed with something that changes the risk. It will probably be years before we have a really good analysis of the complete chain of events.
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