For me it was the worship of the Governing Body. EVERY prayer, SO many meetings and WT pubs would talk abou the Governing Body. I had SUCH dissonance because it felt so wrong. I thought God and Jesus were to be on top, and the GB was just a channel... so why give them so much attention.
I have heard talks that defined idolatry. They said if you give excessive and undue attention to something (like sports, cars, etc) then it's idolatry. So to me the worship of the GB was idolatry.
I of course had many many doubts, but this was the last straw...