Our magnificent D of the first 9 games was showing cracks here and there. In almost every game there was a big play or two against it. Like small cracks in a dam that eventually rip wide open. Teams with better coaches and players simply softened us up by increasing the number of those big plays.
That said, I think the way Tamba and Houston went out of the first Denver game was the most dramatic reason for our defensive collapse of the second half of the season. Tamba came back soon enough, but was he really hitting at all cylinders? Houston had to sit out the rest of the regular season and came back rusty for the playoff game.
The fix, IMO, is (in no particular order) pass rush, pass rush, and pass rush. We need at least a couple of more pass rushers from this draft. Seattle has found gems of all kinds in later rounds. Maybe we can find pass rushers in later rounds of this draft, if not one in the 1st.