If there is any oil leaking underneath the vehicle or white smoke from the exhaust all vans create vacuum behind the rear end as they go down the road. The oil leaks get sucked right up on the rear doors as you drive. If your van is clean and dry on the bottom it's a fact of life they all get filthy on the back-side because if the aerodynamic low pressure on the rear end which pulls garbage up on the windows and sheet metal doors.
Have you ever seen a clean tractor trailer set of rear doors? Same deal applies here.