Probably a static discharge in the case you described. Was it being mixed wet or dry? Dry powders being dispensed from the gate or through a spreader, can be very explosive, especially sulphur. While I haven't seen it, I've worked with others from way back when, who recall cases of it igniting behind them and "chasing" them across the field. I can imagine that to be true.
In a static condition such as a mixing tank, or even in the hopper...watch out! Once it's a dust in the air, it's not just rapidly flammable, but explosive, and can release an enormous amount of energy. Fertalizer, such as amonium nitrate, mixed with sulphur or avgas/jet, can produce a tremendous explosion, or uncontained in the open, a rapid fire. Great care is needed. When you carry those materials, you're carrying around a bomb. Your aircraft is full of ignition sources, as if often the loading pit, with conveyors, pumps, mixers, etc...any one of them could set it off.