NomadSoul, it's an hypothesis. My guess is that a greater proportion of JWs, upon leaving the JWs, become atheists than the proportion of those who leave Christian churches. I'm sure there are plenty of people who go from some Christian faith or other to atheism, but my anecdotal experiences is that JWs are much more likely to make that move than anybody else.
Probably, the best test would be to compare adult converts to various faiths over time. These would be people who took religion seriously enough at some point to convert, so they would be a good sample. You could build an interesting transition matrix, right? Where they start, where they move to, then where they go if they leave that church.
That said, it seems like a very difficult data set to get hold of.
Still, I can think of any number of actively religious Protestants who converted to Catholicism; I'm not sure we have much experience with people who are active in any religion converting to JW-ism. But, I could be wrong.