It has to be something to do with aerodynamics, but why does it collect there?
Many here probably don't leave there cars long enough for it to accumulate, but it happens and I don't know why. It could be the panel gap breaking up the air flow, but then why don't you get it on the front doors? It's not like there is a change in airflow direction afterwards either, as the door flows into the quarter panel.
I was thinking about this before going to sleep last night, so wonderd if any of you lot knew.