IMHO I think Tenet found out some of the stuff that has been going on in the agency to sabotage something and has taken personal responsibility for it. I think the only reason that man would leave his job is if he felt he had hurt the administration in some way. Tenet is an extrememly loyal man. This has something to do with something that has happened in Iraq, either Chalabi, Abu Graib, or some other thing that we don't know about yet. I think he thinks he has failed President Bush in some way.
Fox reported that he wanted to resign before the 9.11 hearings but stayed on to see it through. It was also speculated that because he'd told W that the WMD issues was a "slam dunk", he's lost his credibility.
For what it's worth... I believe he simply thought his seven years of good and loyal service was sufficient, and he wants to get out of that pressure packed job.
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