I was also witness to a nasty BP explosion, whose cause was less easy to foresee. We were at a week-long event, where we were all living under canvas (including Martin, the powder officer). Now Martin had brought a week's supply of paper cartridges in a lockable wooden chest, which he had lined with black plastic sheeting to make it more waterproof.
After the last public display he removed all the unused cartridges and emptied them into a bucket of water to dispose of them.
He then took out the polythene, screwed it up into a ball and dropped it into a nearby fire.
There was a large explosion and a cloud of smoke about 30 feet accross, and Martin staggared out, looking like something out of a Laurel & Hardy scene - smoking hair & clothes, & blackened by powder residue. He had severe burns to his hands (the skin came off like rubber gloves) and he was hospitalised for several weeks while he had skin grafts.
Obviously powder had leaked from the cartidges into the liner over the course of the week.
Martin was no amatuer either - he worked at an arms proofing house.
Looking back with 20-20 hindsight, this accident should have been avoidable, but I think it could have happened to anyone.