For me it was a slow burn. I had always tried to do the right thing and be obedient but always erred on the side of liberalism - e.g. I saw no need to pioneer from school or your level of visible activity being the critical indicator of your spirituality. I also had long standing questions about squaring the pseudo science of the Creation account, the Flood etc with what secular sources were saying.
I served as an MS for many years and it was only my attitude to the ministry that prevented me from being appointed as an elder. I regularly played a game of logic with elders sent round to use unscriptural reasoning to try and guilt me into pretending to do more.
Anyway, there was a point where a member of my family who was a long serving elder was very badly treated and thrown out for a completely stupid reason. This really made me question the real nature of the body of elders and where their loyalties really lay.
From there I started to question if the organisation really was displaying the love I felt Christ practiced. I also started reading Dawkins and others on evolution. That led me to start look at sites like JWFacts. I then got a copy of CoC which I devoured.
I was already aware of the UN scandal but had kind of parked that as perhaps a little overblown by apostates. On second reading however, in the light of the other CoC revelations I was far more outraged.
By the time I started participating on this site I was already well on my way to being completely mentally out. It did not take that much longer before I was convinced the Witnesses did not have the "Truth" and, even more critically, I came to doubt the existence of God at all.
[EDIT] - Oh and then there was all the changes to the generation and other teachings that just seemed to suggest the GB knew bugger all about anything.