It comes from sweeping the range. I ran a range for years. We would sweep it clean twice a day but there would always be a residue on the floor which required a wet wipe down.
Some ranges sweep and put brass in a buckets and those become a huge huge hazard due to the burnt and unburnt powder can still ignite in the right conditions.
It is extremely dangerous in the right/wrong setting.
As memory serves me the Glock Inc incident was from the collection barrel being full and it ignited injuring several and ultimately killing my friend. Just an unfortunate situation of events and the perfect storm.
Most likely in this video that wood flooring was collecting powder from cleaning and shooting. Just happened to be the right time and ignited. It is equivalent to being inside a fire bomb.