The building, or neighboring one, was already on fire.
From Wiki....
“Explosive velocity, also known as detonation velocity or velocity of detonation (VoD), is the velocity at which the shock wave front travels through a detonated explosive. The data listed for a specific substance is usually a rough prediction based upon gas behavior theory[citation needed] (see Chapman-Jouguet condition), as in practice it is difficult to measure[citation needed]. Explosive velocities are always faster than the local speed of sound in the material”
Fire alone does not provide the explosive velocity to explode ammonium nitrate. I think fuel oil is added to increase the explosiveness.