For me it was empty nest as well as some of the changes...couldn't wrap my mind around the "new light" regarding the definition of generation for one. It was also a sense of disillusionment that comes over time.
At first when people disappoint you or you see certain elders behaving badly, you're able to convince yourself that it's a few isolated incidents...we're all imperfect, after all. At first when it seems that every talk and every Watchtower study is a lesson in submission to the FDS, guilt, and do more, you accept it because it's new and you're eager to please. At first, when they talk about the love amongst the brothers, you believe it because they do love bomb you in the beginning and because you want so much for it to be true.
As time passes, though, you are less and less able to keep up the "covering over". You start to see the cracks. The constant barrage of "do more" wears you down until you can't take it any more. You begin to see that their policies in regards to young people don't work very well. You encourage your kids to come forward when there's been a transgression and instead of some understanding or mercy being shown, they reprove or disfellowship them, while the kids that lie are still in good standing and not considered "bad associates". Where is the truth in that? Where is God's Holy Spirit directing the elders?
They are so good at making garbage look like treasure that it can take a while to remove the pixie dust from your eyes and see clearly.