Same as I could say about this bit, fact is, no-one knows for sure exactly WHAT causes it...

i myself own a white gsd who is a 'non standard' colour. i dont afree with breeding purely for colour- my boy came from a mixed litter, he has a lot of well known standard coat dogs in his line and is well bred. the white gene was introduced, not purely for colour, but because it was felt it would improve this line of dogs. the downside (to some people) is the fact that this led to the introduction of whites to this line- only about 10% of his litter was white though- the rest were standard coat. some people say that white dogs have a worse temperamnet than standard coats but if they are raised the same way then i dont believe this to be true.