The dust clings to itself once a thin layer has been established, using humidity or something else as the bond. As it builds, its ability to remain depends on the speed of the fan and when the ability to bond to the substrate has been exceeded, it flies off in clumps. It's kind of like when someone isn't paying attention (or passes out) when smoking a cigarette and they have 2" of ash at the end. It will eventually fall off, but some will stay on the cigarette.