The story where I saw this calls the phenomenon “ground resonance”. While it looks and acts a lot like an unbalanced washing machine in the spin cycle, which doesn’t seem like a resonance phenomenon, from what I understand this can be caused by a shock to the system from landing, and the helicopter is susceptible at certain rotor speeds. If the compensation is 180º out of phase with the wobble you get positive feedback, so it makes sense that this could happen; you’d either want to speed up or slow down the rotor, were it safe and easy to do so, to change the feedback. But this happened pretty quickly.