I've demo'd this effect hundreds of times in AS350. A left yaw produces pitch down, a right yaw produces momentary pitch up followed by pitch down. The opposite is true for CCW rotors.
In my opinion, the cause is flapback as a consequence of RRPM changes. Bear with me here while I offer up some ball park figures.
If the helicopter is yawed, let's say at 30 degs per second, that will give an increase or decrease in RRPM of 5rpm. If yawing with the direction of rotation, the RRPM rises, if yawing against the direction of rotation, RRPM falls. This RRPM change is relative to your position in space, and the surrounding air. It is not sensed, or corrected by governor as the RRPM sensed by the governor is in relation to the aircraft and this hasn't changed.
So when yawing with the direction of rotation, the RRPM rises leading to increased lift on the advancing blade and reduced lift on the retreating blade. This effect is noticed at approx 90 degs due to phase lag and voila, the nose pitches up momentarily. The effect is short-lived as the yawing is short-lived; as soon as the yawing motion stops, the RRPM (in space) return to normal.
The same effect on RRPM can be observed in the hover when yawing left or right resulting in a slight climb or descent.