The way I heard it, there's a small area (volume, really) of gradients of air speed near a fan blade.

The molecules of air touching the fan blade are moving at the speed of the fan blade. When there's dust in the air and it is not moving with respect to a surface, it can "fall" and collect on the surface. We normally think of this as dust falling to the surface, but with a fan blade this effect is increased by the slight amount of static charge mentioned.

A bit further from the blade, there is a difference of speed between the air and the blade, and dust will not collect by "falling," but by static electrical force and by getting caught on the dust already on the blade.

It makes total sense that a higher fan blade speed will fling dust off, since the air speeds at all locations will be higher. The previously balanced system of air and blade velocities will be upset, and centrifugal force (or lack of centripetal force, if you prefer) of the dust particles and clumps will overcome the adhesion to the blade.