http://news.bbc.co.uk/1/hi/england/somerset/8560398.stm

I laughed at the explanation.

"They could have been trying to escape a predator, distracted by light or noise, or maybe even thought that the driveway was long reeds because of the type of shingle is similar to reed seeds"

so basically they don't know and it could be anything lol.

He's an article saying they solved it, but it pretty much says the same as the first.
http://news.bbc.co.uk/1/hi/england/somerset/8609186.stm