I have been completely out for close to 5 years and I have spent alot of time with individuals who either left or are in the process of leaving. Whenever we get to talking about what made you leave or what was it that made the flip switch, the initial answer is some doctrinal teaching. In other instances it was just sorrow or a variety of other things.
However, as you get closer to many of these individuals it was generally a secret sin they committed at the root of it all. They had a habit of watching porn and didnt want to come forward. Maybe they were over drinking but just didn't want to tell on themselves. They slowly stopped praying and THEN began to find fault. After thinking deeply about this the same thing happened to me. I had a real issue with listening to religious music that completely contradicted JW teachings. I got to the point that nothing seemed real anymore. I slowly slide into examining teachings and then things happened.
Many people that I got close to later on realized this was the case with many of them. Some with porn and too embarrassed to tell. Alchohol and the like. THey stop praying and then start questioning. The sad part is I know 3 witnesses who committed suicide. 2 of the three had admitted to the elders that there had been secret sins being committed and they could no longer pray anymore and the embarrassment of what they had done was too much.
THis idea of secret sin is foolish but can lead to serious consequences it seems.