I think it was the pew survey that showed that JWs are most likely to produce atheists (about half that leave remain christian and the other half atheist/agnostic). I also beleive I've read that, of those that leave their religion, where they end up is pretty much a random distribution based on what they're exposed to in their geographic area with the exception that atheism is about twice as likely to be their destination than any religion.
As for why JWs produce so many atheists - i'd say it has something to do with the constant bashing of all other belief systems in meetings. They try so hard to make sure members have no answer to the "where else would I go" question that when they finally do leave they indeed go nowhere. There's also the fact that the JWs train you to spot inconsistencies in other religions, but once you start using that skill on your own beliefs, it's difficult to stop.