It is possible that during the fast rope drill the tail "hit something" and very shortly thereafter pilots had to put it down while still in control.
Other possibility is that while fast roping, something (fired) hit the tail, and once again, pilots had to put it down while still in control.
Other chances are a main rotor blade hit something (building? ) and the whole thing began to shake so they set it down.
Coulda been any number of things.