I'm a bit unusual here because for me it was growing cognitive dissonance that something didn't fit then a few incidents.
I was there at Twickenham assembly when 1975 was announced from the platform. Although young I was old enough to know what was said and that it didn't happen. Some years later I innocently asked my study conductor for the WT from 74 that predicted it and they pretended they didn't know what I was talking about.
I played music in lots of churches as a teenager. I waited for everything the WT said about churches to be true. It wasn't. I did not get struck down, I just met lovely friendly people.
As an older teen I knew I hated being different and hated the restrictions. I faded through my uni years but still had a bit of 'maybe they're right' for a few years. The single biggest incident was really very minor. I was travelling and had spent a perfect day snorkelling off the barrier reef. I did a lot of thinking when I travelled and realised that the world is a truly beautiful marvellous place. The lie I had been sold that it was all dreadful outside the WT, that everyone in it was out to get you just didn't have any evidence. I looked at how beautiful it was and knew that armagefon was not coming any day soon if ever. I think it was the culmination of years of stuff not adding up rather than any specific doctrine. There was n Internet then to redirect you I just worked it out for myself.