It was the problems with how they arrived at the year 1914. I read Carl Jonnsen's (sp?) book on The Gentiles Times and realized that this was really an invention.
If 1914 was wrong, then Jesus did not return invisibly then. If Jesus did not return invisibly in 1914, then there was no appointment of a faithful and discreet slave. If there is no FDS, then the JWs have no authentic religious authority and everything they write and believe is just speculation and in most cases highly questionable.
About the time I recognized that the 1914 thing was wrong, they changed the generation of 1914 teachings. This was the last straw for me as I see that as their first step in a long series of steps to change the significance of 1914 in the JW doctrinal system.
As I have said before, the whole thing comes crashing down like a house of cards.
Jeff S.