Tarmach:
What makes flour explode?
oxygen + fuel + ignition = fire
fire + containment + dispersion = explosion
Grain dust or flour suspended in air is a lovely fuel/oxygen mix. Contain that in a grain silo or kabz' biscuit tin, and a spark can lead to an explosion. The fine granularity of the flammable material allows plenty of air around the fire, to fuel the explosion.
In air air to air fuelling, the two aircraft would be "grounded" to each other before the process began, so that they each had the same static potential, and there is no tendency for a spark to jump from one to the other.