Jon Tenuta calling double or triple blitzes on almost every down. We didn't have the personnel who could make it happen, and a defensive backfield can't cover all of the receivers with single coverage all of the time.
Frankly, the defense actually performed decently when we didn't blitz the linebackers and / or safeties.
Had he simply played a more conservative defense, 4 of those losses would easily have been wins, and 2 more were winnable. At the very least, that should have been a 10+ win season.
In the end, that was Charlie Weis' worst hire, and the one that basically doomed him in 2009.