Originally, I thought someone must have got stuck inthe mud and revved their engine to achieve some traction. However, there would probably have been two tracks of sludge up the front of the vehicles instead of one. I suspect there was a large puddle in front of the cars instead, which had gone by the afternoon because, well, it went all over the cars.
Fortunately (because I got it washed at the weekend), my own car was unscathed. If I'd arrived half a minute later, though...