The tail vane will have a certain natural frequency. The rotor may have a certain mass imbalance. At a certain rotational speed, the frequency of the rotor imbalance becomes the same as the natural frequency of the vane and then the vane starts oscillating. If this is the cause, oscillation will stop at higher rotational speeds. The problem can be solved largely by balancing of the masses of the rotor blades. However, the rotor may also have a certain aerodynamic imbalance if the blades and the blade angles are not identical or if the wind speed for the upper blade is higher than for the lower blade. So some oscillation of the vane will remain even for a perfectly balanced rotor.