They lost (LG) Kelechi Osemele for the season. (injury)
They replaced (LT) Bryant McKinnie in mid-season and promptly traded him away.
So they upgraded LT, but have huge holes in LG and C. LG is being filled by a carousel of backups (none of them good) and Matt Birks replacement at C is a guy they drafted recently, but he's undersized for a lineman and he's getting beat quite often. He's listed at 300 pounds, but reports earlier this year were that he was in the high 280's / low 290's.
Not to totally put it on the Oline, I heard a comment about Rice on the radio a few weeks ago that I found rather shocking. The average college RB usually only gets 100 touches or less during a season. Don't remember the exact numbers, but Rice never even came close to that number. His freshman year was well over 100, his sophmore year was over 200 and his junior year, he had something like 345 touches. They listed off guys currently in the NFL (A. Foster, C. Johnson, etc.) who had less carries for their entire college career than Rice had for his Junior year.
Apparently, there's a little more wear on those tires than a lot of people think.