Ground resonance occurs when an out of balance main rotor interacts with the helicopter undercarriage. This can occur due to poor design or a temporary issue, or a combination of issues, such as a failed main blade lead/lag damper, and/or a wrongly pressurised tyre or oleo leg.
The centre of gravity of the rotating mass becomes divergently displaced until the helicopter either shakes itself to pieces, or rocks from side to side over its undercarriage, so far that it rolls over.
In this situation, as soon as the helicopter leaves the ground, the rolling moment is removed because the undercarriage is no longer in the equation.
In an instance where something big and draggy (like a blanket) flies into the main rotor, the affected blades may well lag back against their damper back stop, causing an out of balance rotor with no damping to help recover the situation. This
could set off ground resonance. If the pilot now lifts off, the drag from the blanket will continue to cause aerodynamic issues. It's easy to imagine why the helicopter will "rock and roll" in the air (once experienced, never forgotten, believe me, I've had this happen).
But how it might also cause yawing as it did in this case, is less easy to explain. Unless it caused so much blade drag that Nr was lost and a subsequent loss of tail rotor effectiveness was also experienced. OR the pilot lifted to the hover before rotor rpm was was up to normal.
Air resonance can occur when carrying a large underslung load, btw. The weight/inertia of the load has the effect of "tethering" the helicopter, in a similar way to the undercarriage when the helicopter is on the ground. Again it would need a "trigger" such as a failed blade damper.
Here's a useful link:
http://www.asl.ethz.ch/education/mas...-Rotorhead.pdf