For me it was a combination of things over a long period of time. Being raised as a Witness, you do things by rote. But when you see sooooooooo many Watchtower inconsistencies, mistreatment of the "brothers", elder abuse, and simple lack of love, it begins to make you start seriously thinking. Then you read Franz' books that show how the leaders of the WT. manipulate people and facts and then there's no denying it that the Organization is based upon deception, lies and and two-facedness. Eventually, all you need is for one person to say the wrong thing to you---and BAM! just like that you're gone!!
But the truth is, it usually takes a long time to get to that point. That's why I believe there are a number of Witnesses that are disillusioned but they're not yet ready to get out.