Ammonium Nitrate is an oxidizer... do you understand what what that means? There are plenty of trucks on the interstate with placards informing us that they contain oxidizers because they can create hazardous situations.
Any oxygen rich atmosphere can be dangerous... remember Apollo 13.
Any oxidizing agent can become dangerous if heated until it reaches a high enough temperature to melt or boil especially in an enclosed space... Hello Captain Obvious, but it doesn't mean its going to explode when it is sitting in a big pile in a warehouse somewhere. If the warehouse catches on fire the oxidizer can and will cause additional hazards obviously up to and including explosive hazards. But this explosion does not fit that profile. If a bunch of morons want to buy the line from local officials to keep their people calm that it was all just a big accident caused by neglect... then that is just fine with me. So what is your point? That terrorists or other malcontents are not responsible? Not too likely no matter what the official line is... that is 25 years of training, drilling with our local military and other Hazmat teams, continuing education and real life experiences responding to hazardous situations after I took chemistry in college.
I withdraw my comment about the concrete truck.