Hi,
Can I try to answer in Alice's absence?
I think it is because that all of the chemicals invoved in manufacturing and treating the carpet are leached out in wet weather and go into your soil
I think from bits I have ready, some can be quite toxic and can be there for years. I am sure someone else can give you better info.
Personaly I am going for manure/card this year. ( If I can find some manure)
Mandy