Why would a vehicle in reverse move forward

This definitely is an internal clutch issue - better head to the transmission shop.

All automatic transmissions have forward bands or clutches that lock up in forward gears (D or drive, 2 or S, and l or low) They are designed to release in neutral, reverse and Park. Sometimes these clutches seize up. What results is a vehicle that will move forward in all ranges. It works fine in Drive because the clutches are supposed to be locked up in any forward gear.

The question is a little vague but I think I know what your referring to. All modern automatic transmissions have torque converters. This is the device that connects the engine to the transmission. All vehicle manufacturers like the trans to be nice and smooth shifting. That's great but it does come with some minor drawbacks. If you are facing slightly down hill and you put your car in reverse but do not step on the gas slightly I can guarantee that you will indeed roll forward. You can eliminate that issue by holding your left foot on the brake while you shift into reverse. Then, as soon as you feel the trans engage, gently depress the gas pedal with your right foot and it will eliminate that forward roll problem.