Seems pretty obvious that it's ammonium nitrate by color and intensity. Whether it was setoff purposefully or not is the question. I asked a bomb expert, whose job is to scout for bombs before POTUS goes places, why in the hell they'd store that stuff there. He said most likely they had no clue how to dispose of it.
Reminder: Ammonium Nitrate port explosion 1947
http://www.local1259iaff.org/disaster.html