why do door lock when you shut the engine off sometimes and get out???

My brother had a situation similar to this. I found the answer was that he had a duplicate key made and in time this key somehow got worn and locked him inside the truck. Fortunately he was in my driveway that night and had his phone, so I got my flashlight after he called me. I saw his frantically waving hand in the tiny corner window that was just cracked open since he couldnt start the vehicle to open the window any more. Needless to say he only used a dealer key after that...this was a silverado Chevy. I rescued him by loaning him one of my Fords while I got his problem researched and solved:) Hope it helps.