This is definitely not the case. NO2 release is characteristic of spontaneous decomposition of nitrates, which can happen under a wide variety of conditions tha

This is definitely not the case. NO2 release is characteristic of spontaneous decomposition of nitrates, which can happen under a wide variety of conditions that have nothing to do with detonation. I’ve had the misfortune of seeing this happen once in a lab, due to a rookie mistake. It usually is the harbinger of a runaway exothermic reaction. There is nothing surprising here if ammonium nitrate is the primary suspect. In the specific case of bulk ammonium nitrate, it has a low melting point (relative to a fire) and it starts to become quite unstable in that phase. The low risk of AN is premised on it being stored in a relatively clean, non-reactive environment. Fuel oil generally isn’t considered a sensitizer for AN, you still need a booster, it compensates for the oxygen richness which improves efficiency. There are a bunch of nitrogen organics and metals, which I won’t list here, that will sensitize AN quite effectively but those aren’t implicated here. Most fires with AN are caused by its reactivity. It will readily combine with other chemicals in the environment to produce compounds that are far more unstable than AN. These byproducts often act as the ignition point that ultimately causes the AN to detonate.