For me, there were a lot of straws and a lot of broken backs.
The first pricking of conscious was learning about the flood and combinig it with what I knew about secular history, i.e. the Egyptians were building the pyramids at that time and show no evidence of catastrophe and reestablishment in their long, well documented history. Learning more about geology led me to cease believing in the worldwide flood for several years as a loyal JW. I started to get suspicious of the bible even though I still believed it was god's word.
The first straw was learning about 607. Seeing that they were fundamentally wrong and then deliberately misquoting archeologists and hiding evidence burst the bubble. I still thought they were sincere though, if misguided.
Learning about the UN was probably the biggest straw. I finally saw the incredible hypocrisy of the religion and abandoned any notion of them being decent and honest.
Learning about Lifton's criteria convinced me they were a cult. Around this time others found out about my doubts and the overnight evaporation of my circle of friends confirmed this. Being bullied by the elders in meetings convinced me they weren't really there to help me.
The contradictions in the bible convinced me it wasn't the word of god.
Learning about the Ugaritic texts and local religion contemporary to the early Israelites led to me abandoning Jehovah as the true god.
Examining all the arguments for the existence of a deity and finding them utterly vacuous, as well as really learning about evolution from reputable scientists led me to atheism.
There is so much more, every time I tell someone the story of my deconversion it is slightly different, simply because there were so many facets and so many things going through my mind at the time.