Is this just a comment on the people I know, or is a more widespread thing on people leaving the organisation?
I think it may be organization-related.
For example, many people upon leaving become curious in it, or, say, Catholicism (which the JWs are also big against), but not so much into Islam or Judaism.
Why?
Probably because of how much energy the JWs spent against Catholicism and witchcraft - those who leave generally do so because they discovered too many lies in the organization, causing them to loose faith. Once they start seeing all these lies, they start wondering (naturally), 'what else has the organization lied about'? The obvious thing to do would be investigate some.
It's not so much rebelling as curiosity, I think. If you were told a car you just bought had a great, new, transmission...that turned out to be junk, and great, new wheels...that had many thousands of miles on them, and were junk, and a brand new cooling system....that didn't work, and had obviously been reworked several times....well....you start wondering about the rest what the salesman said about the car, too, and you have it checked out.