Slippery player

Sorry, this is intentional.

It's not actually slippery, but the windmill acting on your avatar's collision. The collision bounds around your avatar is actually an ellipsoid. It's why you can stand a little of an edge without falling and also why you'll get pushed around by a vertical windmill. Also, you'll notice you'll get forced around near the edge of the wind, where an ellipsoid bounding region would be sloping down. When a downward force is applied on a slope, it can cause a sideways movement if it overtakes the base's friction. Coincidentally, one wind turbine usually isn't enough, which is why the several-odd you added provided enough force to move an avatar while on the ground.

Thanks for reporting.