Those events aren't spontaneous combustion, an ignition source is needed to cause a dust explosion with flour, grain, or sawdust. Spontaneous combustion of something like linseed oil happens with no external ignition source. Spontaneously combustible substances undergo exothermic reactions in normal conditions, e.g. when drying, curing, being exposed to air, or similar. They have low enough flash points that their own heat is enough to cause ignition, which is why you never hear about concrete spontaneously combusting despite having an exothermic curing process.

> which is why you never hear about...

It might help that concrete is not combustible to begin with. Vs. (say) linseed oil definitely is.

Exactly. Grain dust is very dangerous because a small spark can make it explode, but the spark is absolutely necessary. If you leave a pile of oily rags in your garage and wait a few hours, your house could burn down. No spark needed.