For me it wasn't one single thing, but a number over a period of about 16 years.
1) I was 19, a Ministerial Servant, working in the Accounting Department at a Circuit Assembly. The elder in charge and I were listening to the program up in the accounting office of the Assembly Hall. The speaker mentioned a change in alternative service for when brothers were drafted into the military. The "new light" was that alternative service is now alright to take whereas before you had to turn that down too and just go to prison. This elder new many were were affected by this back during the Vietnam War. I saw how deeply this affected him as he chose alternative service as a non-Witness back then.
2) I was in my 30s and my family attended a convention with my parents. On the way home my father shared with me a story about a Circuit Overseer in our home state that intentionally looked for Witnesses who claimed to be Annointed for the purpose of disfellowshipping them. My father shared if we had stayed instead of moving out of state he would have been targeted for disfellowshipping.
3) I was about 35 when my father revealed something deeply disturbing to be. I was 13 when my father moved our family to another state. My father claims to be Annointed and was the presiding overseer in our home state's congregation. When we moved into our new congregation in our new state, that new congregation's presiding overseer told family heads not to associate with us. My father learned that when 3 different brothers came forward and told my father some 22 years after it happened. I then knew why no one ever truly befriended my family and I.
4) About a year later I began to learn why the story of Noah's Flood in the Bible could not have happened from a standpoint of physics. That was the final straw for me.