CJ's correct... lots and lots of causes of coupling.

One thing to mention, though, is that roll coupling is typically linear with sideslip angle, where pitch coupling is typically nonlinear. This means that roll coupling is much more consistent to mix out with a linear mix on your transmitter. I flew Warren Thomas's Taylorcraft (high wing, similar to a J-3 Cub) and he had the roll coupling completely mixed out with a standard linear mix. The ailerons traveled about 30 degrees when you deflected the rudder, but it worked! Slow rolls, point rolls, rolling turns, flat turns were all a breeze. Now, you can also mix out a nonlinear couple with a multi-point mix, but this requires a lot of setup time to perfect, and is often not worth it.