Since there's a lot of snow and the date stamp on this dashcam video is October 30, we can assume this happened in Russia. Also the video description is in Russian. This guy stopped on a single-lane country road to talk to another driver, or relieve himself, or something. Then this happens.
Manual transmissions can slip out of gear! Even automatics can slip out of gear when they get really old and decrepit. Always set your parking brake. We don't know how bad the vehicle was damaged, but the dash cam video was fine. (via Digg)