ISTR it is a function of Blade Vortex Interaction (BVI) - the same thing that causes blade slap on some helicopters when manoeuvring - where the advancing blade smacks into the vortex left behind by the previous blade.
I suspect the relatively low disc loading of such long, wide blades means the vortex dissipates slower (not spat down out of the way quickly).
It certainly becomes quite tiresome flying one - the constant thwack, thwack and the nodding caused by the blades passing over the cockpit doesn't make for a relaxed and noise-free flying environment.