Damn it where's 7th gear when you need it?!
Its an aerodynamic trait of pretty much any car without a boot behind the window, ie estates and hatchbacks. Its caused by the turbulence as the air passing over the roof meets a void underneath suddenly where the pressure is lower. The air is then sucked in from all angles bringing with it all the rain & road muck with it and it gets splattered on the rear of the car. Only thing you can do is drive really slow to reduce the pressure drop behind the car sucking in the rain & muck!! By the way, this is pretty much the reason they dont fit rear wipers to saloon cars as they generally dont suffer from this problem.
God i bore myself sometimes with the crap I know!!
A4 3.0 Quattro Sport - Gone & missed everyday!
Current road going missile.....
........Smart Roadster - arguably the most fun you can have in a car without using lube!!