JW's will mostly leave the organisation because of a personal, nasty or very unpleasant experience. In other words, only the WT can push out their people. At times, we can be lucky to be around to provide the final push or plant the first seed but in my view, the real driver for JWs to leave is the behaviors of the WT (elders, CO's, Bethel people etc). The more personal their bad experience, the more chances they will leave.
Important point. Add GB to that now, with Morris giving his offensive, simple-minded talks.
Every religion has doctrine that can be picked apart, but JWs have the highest turnover because the high control is more likely to create an environment that creates unpleasant experiences.