I'd probably have to see what you're talking about to really understand the problem you're having, but my first advice would be to focus on keeping your energy and your body forward even as you take your body backwards. You stretch your foot to the back, about one foot length or so, no farther, and then take your whole body backwards as a single unit, pushing with your supporting foot, rather than trying to "reach" or "step" backwards. It's that "reaching" effort that twists your hips to the back.
Another mental image is to think of your hip pulling forwards even as your foot stretches back, so that you feel like your leg is being pulled in two directions at once. Also, keeping your hips open will help.
I'm not sure it this really addresses your question, since I can't see your body moving, so I'm only guessing.
Renee