What causes 'wing rock' and how does it work?
It occurs on aircraft that have triangular or arrow-like wing shape. 

Let's say at a high angle of attack, the left wing begins stalling first. The aircraft will turn slightly to the left. Immediately, there are two forces counteracting this: 


when the left wing goes down, it stops stalling and the shadowed wing on the right begins stalling;
if the aircraft is laterally stable, the aircraft has a tendency to revert to the center position when turning left or right.


Therefore the aircraft will turn slightly to the right, and the same process repeats again.