I can think of a few possibilities:
* asymmetric gas flow thru the end gaps producing thrust
* "lumps" on the bore that allow the ring to find "low energy" positions at various orientations as the piston moves down the bore (bore distortion due to cyl pres and/or thermal effects)
* piston rocking combined with uneven ring-piston contact friction about the circumference of the ring grooves
* coriolis effect (a stretch, I know, but pistons do undergo significant acceleration, and I don't remember my kinematics very well)