Bad defensive play starts in ripples and ends in waves. If you have average players, they'd better play perfectly, understand their role within the larger defense and tackle reliably. Every player who does their own thing, doesn't play with technique or simply lacks the tools to do their job creates ripples that eventually cascade into a tidal wave that rolls through the defense.
Against Cal, it started up front, spilled through to the linebackers and eventually reached a pair of not-ready-for-primetime safeties who were left trying to stop the flood with a tool kit of Ziploc bags and bathroom caulk.