Just a wild guess.
The Robinson has five 'hinges' that will accommodate vertical motion of the two blade tips.
There is one teetering hinge.
There are two coning hinges.
The two blades can structurally bend in a vertical direction.
This is more flapping/teetering 'hinges' than any other helicopter has.
The degree of teetering rotation is probable comparable to other 2-bladed helicopters.
The coning hinge will allow an additional 3-degrees of downward movement over that of fixed pre-coned hubs.
What is left is the flexure of the blades and the clearance between the horizontal rotor disk and the boom.
Might it be that, during an unusual situation, there is a self-excitation that takes place amongst all these five 'hinges' and this causes one of the tips to drop excessively low for a portion of one revolution?
Just a wild guess.