I'll weigh in with a guess. As the gen 2's underwent a design change, its possible that in order to make the body more aerodynamic they created a faster slipstream one the upper portion of the vehicle than the lower. This would cause the upper area of the doors to be operating at a lower air pressure and causing a partial vacuum to exist (lower pressure at the top or middle of the door in relation to the pressure at the bottom. (I'm thinking only 1 or 2 psi difference).
This might then cause a circulation of airflow, entering at the door bottom carrying the debris (dirt, dust, salt) along with it. If this theory is correct, you would need to find a way to interrupt the airflow.
I looked on my 09 MKX after we got back running around today (streets covered with snow, slush and salty water) and basically nothing. So my guess is an unintended air flow design change is the (possible) cause.
Edited by enigma-2, 30 December 2016 - 09:20 PM.