In my case it was several things. About 10 years ago, I read on some website (don't remember what it was) about the Mexico/Malawi fiasco. The more I read, the more alarmed I was, until cognitive dissonance kicked in and I stopped. Even though the questions and scary thoughts lasted for months, eventually I was able to let my "wait on Jehovah" knob be cranked all the way up.
However, questions about several things and never cleared doubts kept nagging at me. For three years, about that time I was disfellowshipped while being a MS and had the common experience of thinking "well, if I'm going to die in Harmaggedon anyway, do as you please while you can". That lasted about 3 years and then I couldn't bear it anymore and went back, became an MS again and then an Elder. A week after my appointment as an Elder, the NBC news of child abuse and JWs came out. On top of that, the KM came out with that article about not reading other literature, not learning greek or hebrew and just reading the WTS information.
At that point a huge light bulb went on in my head. And the question "What are they trying to hide?" kept nagging at me. So, I gave myself permission to check everything about the organization's history, it's critics, read CoC and Christian Freedom and other books and discovered what was it that they were trying to hide and it was a huge lot!!! The rest is history.
Vivalavida!