It it's pure uncontaminated form, it's extremely stable. It's extremely stable when it's just AN and it's extremely stable when you make it AN+FO(fuel oil) ANFO. It's not even considered an explosive. It's a blasting agent and it requires a detonating agent to even set it off(1 lb primers are used most often with a blasting cap setting off the primer).

I'm skeptical that Ammonium Nitrate that had not had any oil(kerosene or diesel) added to it could go high yield like that did. It might just be the news using the wrong term but pure AN is not explosive. In other words, that stuff was already ANFO, sitting in a warehouse for years and became unstable and unpure over time which is a possibility. I have no experience with this because when we got ANFO and put it in our blasting silos, we used it quickly. This kind of thing would never happen because ATF would freak out. Or someone mixed the fuel oil into it and arranged it so that something was used as a detonator to set it off. Fire typically does not set off ANFO - not saying it's not possible but it's not typical.

Also of note the red smoke you see is poorly combusted ANFO. When it goes undetonated and burns it gives off that orange/red smoke. Very toxic.