Having been in a ******* in which the HP lost tail rotor control I think I have an idea on why this happened. The statements put forward by eden were borne out in this instance.
The HP was a young pilot (tw@t) who was not as good as he believed and who had a history of over-controlling. 2am over a well lit city to the W of GB ;-) We were hovering downwind (in limits, honest). Young chap decided to lose a bit of his limited concentration and sent us backwards. Wind crossed from one side of the tail to the other, then under the horizontal stab. The result was a spectacualr >360 which included a healthy dose of 80 deg nose down.
The next day he was heard spouting on about Fenestron Stall until it was pointed out that his poor handling had intitiated the whole sorry affair.
Regards PFL