I was reversing. I exited vehicle The Car was either not in park or fully In park. I got something from the back seat, turned around and the car with drivers door wide open was barreling towards me. I barely managed to get into the car. I do not think tjr car was reversing when I reached into the backseat. It appeared like it was a delayed reaction. The car door was damaged as it hit the side of the garage. I am so grateful nobody got hurt. Do not want this to happen to anyone else. (I run and miraculously made it inside the car. Most would not have.