My wife's car does this also...just a matter of how "loose" the torque converter is. By loose I don't mean there's anything wrong...it's just a term used for a converter that has more torque multiplication than a "tight" converter. The "looser" converters don't roll backwards because they have enough torque multiplication at idle to hold the weight of the car. "Tighter" converters are better for gas mileage, so you see a lot more of them now. Electronic engine management and drivetrain controls have also allowed for "tighter" converters than could be used in the past.