Bob Loblaw pretty much nailed it. With little pass rush (until the end of the season), no outside containment in the run game (OLB's were in a pin ball machine....scheme) not keeping the o-line
off your best linebacker (Tinsley), it's not surprising the secondary (not just the S's) had to be
heavily involved in the run game.
The biggest improvement on defense last year was made by the defensive line. As a group, they (mainly the ends) were absolutely horrible at the start of the season. As the young ends gained experience/got better, the defense/team got better. As a result guys likes slim shady, Vereen and C.Lewis were trashed last season because they were asked to do things that guys who were drafted in the 1st round weren't asked to do.
If Vereen got "turned around a lot it was because the offense had sooooo much time to do it. The USC game says it all; 2013 1st rd QB had his blind side protected by a 2012 1st rd LT who was blocking a 19yr old 220 DE making his 1st start and the QB was throwing to a 2013 1st rd pick WR. No DB who played college football last year could have done much better.
Vereen is a very good football player and cb; he was not the issue with this defense last year...he was a victim of it.
Note: the young guys on defense had no business being on the field last season and the coaches had no choice but to play them. To the youngsters credit they made a ton of improvement as the season went on.