I flew one for about ten years and have never experienced the aircraft pitching up uncommanded.
Though having said that, I flew with a chap who did, and in recovery over pitched nose down. In recovering from the pitch down he took out part of the vertical stabilisers with the main rotor. (He stated the nose up movement felt like going vertical nearly rolling onto it's back)
The pilot involved gave up flying after the experience.
He also associated it with turbulence.