This is coming from pretty high from a source I trust..not that it's a huge revelation. But JD quit because he knew Stankey was taking over, and he really doesn't care for Stankey one bit.
This is just because some of the other choices - that he had health problems and that he was "forced" out - may give the wrong impression. If you listened to JD, especially in a smaller setting, you might agree with me that he was a pretty stand-up guy, relatively humble given his position. No wonder he didn't like the bloviating blowhard that is Stankey.