I think for those who leave it is a process. The initial 'doubts' will not cause one to move ... it's a series of events and/or set of circumstances that gradually breaks the spell.
I did leave due to my personal growth coupled with experiences while in the tower and personal experiences with non-dubs. When I realized that my family still loved me even though I chose to associate with witlesses instead of them; and that some of my 'worldly' co-workers cared more about my well-being than the dubs did, it helped to readjust my views on worldly people, thus severing the cord that tied me to the b0rg: THE FRIENDS
When the 1914/generation light flickered, it was for me the third time the b0rg changed directions - I knew in my heart right then that they had no idea what they were talking about, and that all the Armageddon gibberish was just cult BS!
I think it took a period of about 4-5 years in all for me to actually leave. But each small awakening and every critical thought that followed was another nail in the coffin of cult membership. Of course I kept my doubts to myself, not even mentioning them to my husband at the time (or since). Strange how the prohibition on discussing one's doubts seems to make them grow stronger. Once I began to doubt the truthfulness of the society then I began to act more independently. I was one of those tow the line dubbies, turning in honest field service reports and ratting myself out for disobeying mother . So for ME to ACT contrary to their teachings was disassociation itself.
Before I even thought of leaving my two brothers told me it was a cult (did I believe them... NO). A couple of my mother's friends warned her too but I didn't listen. By the time I was ready to leave everyone in my family was so surprised they didn't know what to think but they helped me and my children to get on our feet when my long time spiritual family turned their backs on us like we never existed at all.