Why does a dust explosion occur in any empty grain silo while if the silo is full of rain only a fire occurs?

Fire needs heat, fuel and oxygen. An empty siloh has a better fuel/air mixture and every piece of fuel has more than enough oxygen to burn, and so it all burns very quickly and expands fast enough to be rated as explosive, whereas when grain is stacked solid in a full siloh, air has to get to the fuel before it can ignite, so only the top layer exposed to air burns.