It really is a multi-part question for most people. There is an accumulation of issues that finally becomes too great to accept anymore. There are a small handful of those issues that one finds most significant. And finally, there is a "last straw" that opens one's mind, allowing him or her to really consider the issues that have been stewing on the back burner all that time.
For me, the last straw to break the mental grip the organization had on me was a meeting a couple elders had with me, counseling me about questioning decisions of the body. In that meeting I was told by this pair of men that to question them was to challenge Jesus Christ himself and that thinking I knew better than the spirit-appointed body of elders indicated my character had the flaw of pride.
The worst thing about their view was that it was the organization's view; they didn't just make it up. They showed BorgLit and twisted scriptures to support their viewpoint.
So even though the thing I had criticized was my professional area of expertise and nobody on the body had any experience in this particular field, I was portrayed as wrong for questioning.
It still took me a few years to get physically out but from that day on I was able to look at "apostate" books and websites guilt-free. My mind was freed; the Watchtower's way was NOT God's way.
But looking back, that final straw was just a culmination of a lot of little things, and the beginning of a subsequent education about "the truth" that informed my decision to finally leave and never go back for any reason.