Ammonium nitrate doesn't go off that easily without prep, but it can if there is a fire, especially in a reducing atmosphere which isn't that difficult to achieve once a smoldering fire gets underway. It could be some form of gross negligence, it could be intentionally set with or without the intention to get the entire thing to blow like it did. Really way too early to say.
It doesn't act like there was a bunch of ordnance hidden though, an ammo dump going up is a fireworks show as stuff gets thrown into the air and blows up there or where it lands, this was one big continuous boom. Before the big one there looked like some bunch of smaller explosions going on in a kind of random disposition but that could still be consistent with a huge stockpile of ammonium nitrate working up to a massive det.