Thats an easy one. Remember that the tail rotor conteracts the torque from the main rotor. With the push of both these forces the body of the heli leans to one side. This happens with real helis too only it isn't as noticeable since their weight keeps them perfectly straight. It is a perfectly natural thing for the heli to do. You could try turning the tail rotor a tiny bit to redirect the thrust to counter the lean, but it will still be visible. If you look at the rotor disc during hover you wil see that it is sitting level even though the heli is leaning.