Their defensive line and linebackers, for the most part, were really big and slow. That's their problem. Against most teams, it doesn't matter, but a team that can come close to matching them physically and has a ton more speed on the edge, they lose.
Thats what happened. If anything, it starts with recruiting quicker Lb and DE, much like how Urbans teams are built--strong, athletic, quick D linemen instead of just massive 320 lb hogs.