It was many things:
Reading that the Watchtower had joined the UN, with the excuse in the Kingdom ministry that they needed to use the library. It's not a big deal now, but when I joined back in the late sixties, the U.N. was considered an evil thing. Considering how the religion instructed JWs in Arica to die rather than buy a political card, it seemed weird and hypocritical.
The realization that you could never, ever do enough to not feel guilty and that this was deliberate. You were expected to to preach as much as possible, but the preaching work was ineffective and futile and just a big waste of time.
The complete lack of warmth of caring in the congregation, and feeling marginalized because my husband was disfellowshipped.
The straw that broke the camels back was looking back over my life and realizing that the advice I had been given, to stay in a bad marriage, was really, really terrible advice and had done nothing except make me miserable and give my children a bad example of what marriage could be.