Something malfunctioned with the brake system of the car. The car was in park and off. I exited the car, reached back into the car for an item, and shut the door. I then walked up the driveway and to the front door. At that point, the car began to roll down the driveway. Someone coming down the road in front of the driveway hit it on the front passenger side. The car then continued to roll across the street and part way up the driveway across the street, coming back across the street and then ran into the same car that hit it. I was then able to open the car door and get inside. It was still off and in park. I depressed the brake pedal, started the car, put it into drive, and moved it to the side of the street. After the incident, I tried multiple times to replicate the incident with my car. I tried leaving it in reverse, even though I know it was off and parked. I also tried leaving it in neutral and turning it off, even though I know it was off and in park. Each time, the car began to roll immediately. There was no way that I could have done what I did that day, which was to step out, reach back in to get a bag, shut the door, and then walk away. The car was definitely off and parked when it happened. The vehicle is currently at a Mazda authorized repair shop, and I am awaiting the final word on whether or not they can detect what exactly caused the brakes to malfunction. There were no warning lamps or messages prior to the issue; however, I did notice that something s