It happened to me twice, and when I asked about it a few years back everybody said it was highly unlikely in the robbie. Nick Lappos finally answered my question and then it made sense. If the wind is off your 10 - 11 o'clock position in the hover at high gross weight the main rotor tip vorticies can get into the tail rotor and disrupt the flow and thus LTE. I've since been told that it can be over come with left peddle but I probably didn't put in FULL left peddle and have since never experienced it again. Although now maybe with the onset of it I use Full left peddle and keep a closer eye on the wind direction.