Likely a tail rotor or tail fin, very common to have failures of the 42 or 90 degree gearboxes or the fin itself in ops like this. You have to consider that in normal helicopter operations, the gear boxes of the tail rotor for anti-torque don't get worked that hard since the aircraft is often in forward flight. In ops like logging or heavy lifting, most of the time is spent in hovering operations or speeds below ETL, where the rotor system is bearing the brunt of the load, and the tail rotor gearboxes are working hard managing anti-torque when there's large power demands being made of the helicopter.....all with no aerodynamic "help" of forward flight. These kinds of failures are very common in logging operations, and are a high risk to take, since you're already essentially a dead man walking, by the time the failure occurs: you're in a hover, normally over a place you can't land, you can't get forward flight, and you have to immediately enter an autorotation in order to keep from spinning...so you're going down. THAT is all assuming that the mass CG shift from losing a tailrotor, or rotors, or a gearbox, or gearboxes, or the entire tail section......doesn't cause a massive forward CG shift and nose you over into the ground. Further, where the gearboxes attach to the tail fin area, there is high fatigue to the fin component due to the high stress they're constantly under. The FAA even issued an AD (97-20-09, and two amendments) specifically covering tailfin overstress fatigue during these types of operations.