It has nothing to do with your seat belt.
Did you open your door to check your positioning while you were backing up? It won't let you do that--the car will do exactly what you are describing. (or was your door just not fully closed?)
2014 328i xDrive | Black Sapphire / Black Red Dakota | Sport Line | Premium Pkg | Lighting Pkg | Cold Weather Pkg | Driver Assistance | Nav | Sport Automatic Transmission