Can anyone explain to me what causes wing rock during high alpha flight and what you can do to an aircraft to reduce the tendancy? Why are some 3D capable aircraft more prone to it than others?

Cause of wing rock? Well we'll leave that to the aerodynamic big brain guys.

Wing loading is probably the biggest factor.
Lighten a 50cc plane by a pound and it will wing rock less. Just a few onces on a .40 size plane makes a big differance.