In Seattle's last 20 games, they've been really good at run defense with three exceptions: Tampa, @STL, and Atlanta.
The RBs in those games? Mike James, Zac Stacy, and Michael Turner. James barely registered before that Hawks game and was injured soon after, Stacy had 1 YPC in the rematch, and Turner was horrible in 2012, he did not play a down in the NFL in 2013.
The common link between those three RBs is that they are decently physical, and during those games our LB crew couldn't tackle anything. James and Turner in particular seemed to break a tackle on every single run.
The fact that this happens rarely, randomly, and emphatically- always against mediocre RBs- is further evidence in my mind that it's a result of our LBs just having off days.
I thought our LBs looked pretty bad in run defense in this Oakland game too. Though to Oakland's credit, some of those issues with run fits / gap integrity was the result of good fullback play.
I think it is part of the cost of doing business when you have undersized LBs who play so fast. Every now and then, a fast enough, physical enough no-name RB is going to put clownsuits on them.
Maybe there is more to it than that, maybe in those games our guys up front aren't doing enough to keep our LBs clean, I'm not sure. But I do remember that in all three of those games, missed tackles and poor play from the second level is what ignited the other team's rushing attack.