BVG seems to have the same "stubborn" quality on D that BK has on offense. As the injuries piled up, he continued to try to do the same thing with back-up players. His defense became predictable after a few games and by continuing to use the same schemes with a depleted defense, it became even more predictable and less successful.
I think if our D had remained healthy, the predictable nature of our D would have hurt us, but not to the extent we saw.
Combine those factors and BK's offensive scheme that put the D back on the field too often and you get the shit show that was ND football since their return from Tallahassee,