Q: Car lunged backward 10 feet in parking lot

Recently I was parked in a large parking lot and when I got into my car and turned it on, it lunged backwards. Instead of backing out of the parking spot, I decided to make a call using my bluetooth. That took about 10 minutes and then after hanging up, I released the emergency brake, moved the stick shift to Reverse, and had my foot on the brake. When I released the brake, the car lunged backward very hard, about 10 feet across the parking lot where other cars could have been parked or a pedestrian, but fortunately were not. I immediately put my foot on the brake, stopping the car. I don't have any idea what caused the lunging of the engine. I've driven Toyotas for many years and never had this before. When researching online, I deducted that it might have been the power train that was the problem. Do you agree or is it something else and what should I do?