Here's a theory about what the "personal reasons" might be. Let's say you're the DCIA and you've been hearing strong and persistant intel about a potential devastating attack on the US especially in the mid-July - November time frame. Your network has not been able to surface any specifics and the clock is ticking. What should you do? If you stay, you could be, at a minimum, blamed for the attack or, at worst, you could be one of the victims. One telling sign will be where Tenet moves after his resignation. If he stays in the northern Virginia area, then the theory is probably untrue. However, if he moves out of that area to someplace, like say, Arizona or Nevada or Montana and does it rather suddenly, then the theory might have more credence.
indeed, I posted this earlier. who exactly is going to track whether Tenet moves his family to Montana?