No way that a spark would set it off. I did once work for a blasting company and more than once have used ammonium nitrate prills to help start a fire in my wood burner. The stuff burns great but it needs something more energetic to make it blow, something like a stick of dynamite. Also, you generally don't mix it with other compounds until you are ready to use it. You can get it pre-mixed by the bag but usually, they spray it with diesel as it goes in the hole. You use a cap to set of the dynamite with one stick in each hole. The rest of the hole was filled with ammonium nitrate that had been sprayed with diesel fuel and then stem it to seal the blast. It always blowed up real good but no dynamite, no bang. It just burns. It burns real hot. It would be super easy to determine if it really was ammonium nitrate as it is one of the slower explosives out there at only 2700 meters per second. Just play the video in slow motion and see how fast it spreads.
“Also, you generally don’t mix it with other compounds until you are ready to use it.”
They mixed it at the plant and took it to the mine in containers to pour it into the blast holes. The plant was no bigger than a common sized machine shed.