Definitely, as with others, cumulative for me. What OnTheWayOut and jp1692 and Sunworshipper805 wrote applies to me.
I had doubts even before I was baptized - doubts about such things as the flood account and the JW teaching that animals didn't kill each other before the rebellion in the garden of Eden, but I suppressed them. Very early in my JW life, I saw flaws in JW higher up personnel, poor writing in JW publications, flawed doctrine, etc., but I still thought JWdom was what it claimed it to be.
The 1995 "generation" change took a lot of the wind out my sails. After that, I just couldn't ignore anymore the fact that the ministry and the meetings were largely a waste of time. Most JWs were not effective in the ministry, the meetings were mostly fluff, and most of the appointed men were not good teachers.
Also, I, as did OnTheWayOut, realized that the org was making money on disaster relief, and, at the same time, bragging on itself for how noble it was. Then, the fact that we were getting further and further away from the year 2000 (kj page 216) with no end in sight weighed heavily on me.
Finally, after much prayer, I slowly began researching on the internet. Found out about the UN thing, the deception in the Creation book and the Trinity brochure, and lots of other stuff.
At some point (2012 or 2013?), it seemed like scales fell off my eyes. I sat at meetings in the back and just watched and listened. It began to seem so corny and goody goody and empty and undignified and cult-like.
Then, the straw that almost broke the camel's back... the last Memorial I went to. The talk was not at all serious and dignified as it seemed to me a Memorial talk should be. The elder talked about how in the "new world" animals wouldn't kill each other and he used a silly example (can't give specifics because doing so might identify me). The JWs chuckled and laughed and ate it up. I and my wife did the opposite of laugh.
Then, the final straw. Shortly after the aforementioned Memorial, we went to a regular Sunday meeting. During the Watchtower study, in the audience was one of the local elders - a loudmouth who could barely spell his own name and knew nothing of JW history and very little of JW doctrine current at the time. He commented in a boisterous, arrogant way during the Watchtower study something like the following: "Don't even be curious about apostate websites and words. All it takes is a few words to wreck your faith."
Of course, I had heard that before and, I think, read it in JW publications, but this time, it just hit me hard. It just made me really realize how closed-minded and overly confident and sheltered and cultish and ignorant JWs are. I was thinking "If you have the truth, what the hell are you so scared of? Is your teaching that weak? Is your god that weak?"
Haven't been to any kind of JW meeting or event since.