A few different sources combined. A work colleague told me something he thought the JWs believed that sounded nuts, so I looked it up to prove him wrong, and of course he was right.
Other things like animals being vegetarian and learning to kill from man when some are perfectly adapted killing machines and would never see a human being kill anyone (living remote or under water for instance) made me question things.
Ultimately, the elders response to questioning was the final convincer - they didn't have answers, only threats not to ask questions.
Sites like freeminds and others were useful sources of information once I'd started questioning things and looking for what was wrong, and of course books like Crisis of Conscience and just general learning about history and the many myths and fanciful stories in the bible itself and the practicality / stupidity of some of the stories (Noah, Sampson, Jonah et al)
Oh, and I didn't want my children to have the upbringing I had.