Lots of things can wake someone up....too many to list. However, while I dislike to just cut and paste links, I think this one explains well while despite overwhelming evidence of falsehood, even intelligent people cannot easily break free from any religious belief.
The article mentions four key points:
1. Intelligence is compartmental.
2. The earlier someone is raised in the belief system, the more difficult it is to overcome.
3. Christian religions in particular drill into believer's heads to distrust themselves.
4. Psychological and social fears of questioning prevent one from facing their cognitive dissonance.
I would add that in the particular case of JWs, the degree to which all four points affect a person, are much greater than the mean, and in some cases a positive feedback loop is created, making it that much more difficult to break free. Those that do break free generally do so because the positive feedback loop in of itself tends to cause instability, which typically translates into some personal event that causes the awakening.
d4g