Ah - well, in the first half of the book (its written as a series of documents) there is a scientific report in which a specific (terrestial) cause is speculated - it nicely leaves the door open to speculation along the lines of 'was it an accident, a weapon, terriorist action, etc.'

As interesting as the cause is though, the story was strong enough in my opinion without it - it wasn't a story about the snow, it was a story about how people coped afterwards, if the cause had been revealed in the conclusion then I would have been happy, but I was more concerned with the fate of humanity and the characters of the story in general.