It was an elderly man driving. He had parked outside of the post office in the small town in California, where I lived. I was in a store next door, when I heard the crash. He had jumped the curb from the parking lot and his car was halfway inside the little post office. I was the first person on the scene. The man was not hurt, but was somewhat confused from the accident.
I looked in his car as I was asking him if he was OK. The transmission was in Drive.
I know exactly what happened. He meant to back out of the parking spot, but mistakenly put the car in Drive. When he pressed on the accelerator to back up, the car didn't move. So, he pressed harder. That provided enough power for the car to jump the curb and hit the post office. It was very simple. It was something he did completely by accident, and the additional pressing on the accelerator was his response to his error with the transmission.
I suspect that is the case in most of these parking lot incidents. Transmission in Drive instead of Reverse, no movement of the car with a light pressure on the accelerator, so more pressure is applied, until the car does move...in the wrong direction.