It is my understanding that he was burning some unused (and old) powder(s) at his home and the powder(s) "flashed" causing him to sustain burns over 70% of his body. I have not seen any disclosure of the amount of powder he was reported to have been burning but it was implied it there were several pounds. I also have not seen any real definition of what caused the "flash" to occur. I have been loading shotgun shells at the same bench located in my garage for 15 years. I have carpet on the floor in that area. I don't spill much, but admittedly there is some small amount that does hit the floor. The only reason I sweep is to keep the general area clean. Smokeless powder does not explode like black powder. It merely burns unless under pressure and ignited by a source (i.e. a primer)
Nonetheless, general safety precautions are always adviseable.