This happened some time ago in Auckland New Zealand. If I remember correctly, the helicopter was lifting the tower in the left of the picture. The helicopter hook failed to release due to a malfunction. The helicopter hovered lower so the ground crew could release the cable. I understand that the lifting cable had sufficient length to carry out this manoeuvre. Unfortunately the ground crew in their attempt to release the cable from the hook, pulled the cable into the rotor system.
I might also add that the pilot was an experienced operator.
As usual in these types of accidents, its always the unexpected that gets us.