I was at a 24 hr storage unit. It was dim & I needed to unload from the rear hatch door so I backed to the open storage unit door in its upright position as high as I could get it & stopped at a safe distance from the doorway so the car door would not catch the storage unit door. I put the vehicle in PARK to have some lighting & double checked before I got out- gear shift securely in PARK. I opened the car's rear door & started unloading without overly disturbing the vehicle or causing any unnecessary movement. I had just turned my back to the car door to put down some boxes in the unit. As I turned back to the car to retrieve another item, I noticed movement. There was a rapid simultaneous series of events: I felt the rear hatch hit my nose, I heard a crash with shattering glass & I tried to jump clear. It was over in minutes . I was inside the storage unit and had to squeeze out to try to free the storage unit door. The door had fallen and gotten wedged between the rear frame of the car & behind the back wiper. I gently freed the door after much diigent effort. Once freed, the car proceeded to move forward on its own before I could access it to shut off the engine, & it eventually crashed into the opposite storage unit. I accessed the vehicle at that time & reversed away from the buildings so the car was equidistant from both buildings & facing an area where it would do minimal harm if it moved again, though I did cut off the engine. Verified vehicle was in PARK. The car