My opinion: I think it takes a "perfect cocktail" of circumstances.
- Personal mistreatment by other JWs causing you to question the "Love" in the congregations
- Missing meetings and "theocratic activities" is KEY. Without the constant mind numbing cult bullshit, your mind and body can have the breathing room and reduced stress to actually start thinking for yourself again.
- Addressing doubts instead of letting cognitive dissonance take over. Giving yourself the permission to question and use your god given brain (if you believe in him/her/it) to apply real reason to the teachings.