Ground resonance. It can occur anytime the skids are on the ground, but it's also possible something mechanical allowed the spacing between the blades to shift out of tolerance and thus shift the center of balance for the rotor disk. Rotor blades may, depending on the type of rotor system, lead and lag (move forward and backward) in the plane of rotation to compensate for acceleration/deceleration. If a blade is allowed to shift too far (if an elastomeric dampener fails, for example) it throws everything out of whack, technically speaking.
The old video of the CH-47 tearing itself apart while tethered to the ground is the same thing.