There's a basic principle about the ratio of surface area to internal volume - grinding something into a fine powder increases the surface area dramatically, giving more area for chemical reactions. This basic principle is seen frequenly in biology (a limiting factor for the size of a single celled organism is the surface area to transport food to the internal volume), chemistry, physics, and even solid rocket engines.

Take a lump of coal and touch it with a burning match. Odds are nothing will happen. Grind that coal into a course powder and it'll light much easier. Grind it into a fine dust and it may well explode. Back in the late 19th and early 20th century, most steam powered ships were fueled by coal. There were quite a few explosions. Between the tendency for coal to spontaneously combust and the explosive nature of coal dust, it was a very dangerous situation. Hyman Rickover believed that the explosion of the USS Maine in Havana Harbor (1896) was likely the result of spontaneous combustion of a coal bunker that was near the ship's powder magazine.

When I was stationed in Nebraska, every farming town had its grain elevators next to the railroad tracks. It wasn't uncommon for those things to explode if something ignited the corn dust. Back when the Reagan administration was proposing moving MX missiles from silo to silo to defeat Soviet attempts at a preemptive strike, I suggested the Air Force lease a tube in a grain elevator in every small midwestern town. They could move an MX missile to the town in the middle of the night and use a crane to load it into the elevator. The MX was cold launched (compressed gas shot the missile high above the sile before ignition), so fratricide from the missile igniting adjacent tubes shouldn't have been a problem. The Soviets would've had to target every small town to be sure of getting all the missiles, and even they didn't have that many warheads. The leases could help the towns' economy, so it likely would've been popular with the townspeople. Obviously, nothing ever came of my suggestion.