Let's look at the pictures, shall we...
The damage is confined to an area at the back of garage or the room behind it. The resulting damage indicates an explosion but not much (if any) of a resulting fire. This is backed up by the fact there is lots of small debris and a partial collapse of the truss system over the point of origin but the rest of the house remains intact with no smoke staining or sooty runoff. In fact, there is no water or sooty runoff from the fire department down the driveway, where you would expect it to be. Witnesses also say they heard "an explosion".
This was not a natural gas explosion (from a gas leak to a clothes dryer or gas water heater, either of which you would expect in that area of the house). If it was, the damage would be much (MUCH) greater - with possible secondary damage to surrounding structures. The same can be said if it was a meth lab - plus there would be more evidence.
The news reported that investigators already made the determination that it was a result of the reloading activity and the smokeless powder. There is nothing shown that contradicts this conclusion. I would not be surprised if the homeowner/reloader may have had a fan or an AC unit running in his work area that may have stirred up some powder, or possibly tipped over his container by accident. Get enough of it airborne and it WILL explode. Heck, wheat dust will explode. I also wouldn't hesitate to believe that he may have been smoking or was using some electrical equipment that may have generated an open spark.
The most amazing part about what I see is that he is actually using his garage FOR A CAR!
Never underestimate "Florida Man"...