vortecy is on to the reason, I think.
The helicopter has a balance of pitch axis forces, with the fuselage, the horizontal tail and the rotor (through longitudinal cyclic) all part of the sums. In a sideslip, the tail and fuselage balance is upset, and the horizontal tail can be reduced in effectiveness due to the angle of sideslip and also if any fuselage blanking takes place. Blanking means that the tail can be placed in the wake of the fuselage, which reduces ite effectiveness.
This can strongly effect the need for cyclic pitch to rebalance the aircraft, so much more flapping can be needed.