Oxidizers can explode under the right circumstances -
See West Texas or Galveston where bulk AN detonated when exposed to fire. Some reports are saying it was sodium nitrate (more consistent with the color of the smoke than AN) and that is also a strong oxidizer known as saltpeter. When there are large amounts it creates confinement just by the size/weight of the pile and if you expose it to fire bad things can happen.
Saltpeter is actually potassium nitrate. Stump remover, and to my knowledge, cannot go off like Ammonium nitrate, which has the ability to transition from deflagration to detonation. According to wiki, this can occur with impurities.
It’s evidently similar to the Ammonium perchlorate rocket fuel, which can do the same (look up the Nevada rocket fuel plant explosion).
It will be interesting to know if this was a AN dump going up, or was it a Hazbollah ammo dump going up. Hezbollah sure was quick to deny involvement!
Old Saltpeter, 3-2-1, Saltpeter, sulphur, charcoal = Black Powder. Made it myself.