Bike fit / correct riding position

Train yourself not to do it if the motion is only very slight. Watch your knees on the turbo and make sure you're tracking them straight, correct the motion if not.

I do the same thing and my bike fitter told me to do hip abductor strengthening exercises with a swiss ball to help beef up the muscles that control the lateral movement there.

This is what I was thinking. Pretty sure its just bad habit.

I do a bit of swiss ball training at home. So I could incorparate that too. Although I'm about to have horrendous acupuncture on my right hip joint. Maybe I'll wait to see what that does.