2007 Mercedes-Benz Sprinter Rear Door random automatic locking???

Hi,

I have a 2007 Mercedes Benz Sprinter MWB 311CDi and every time i unlock the van or stop and try to open the rear door its fine, until I close it and come back to open it again. It for some reason locks itself and requires and unlock with the key or the button in the cab! Is there and way of disabling this random auto lock???

Thanks, John