http://www.nfl.com/gamecenter/2015091301/2015/REG1/seahawks@rams#menu=gameinfo|contentId%3A0ap3000000529256&tab=videos
There was a big hole between the left guard and left tackle, the Rams LBs got sucked up and left that gap wide open. Would have not only been a first down, but probably a HUGE run if Lynch hits it. But he would have needed to hit it right away. Lynch hesitated, Donald slips past Nowak after being blocked for a tick, RT Gilliam got owned by his guy too. Game.
It was one of the few mistakes Lynch made in this game. He will usually read those correctly. I think Lynch decided very early in that play that he wanted to follow Sweezy, and for what it's worth Sweezy did do his job. The two guys around him, no so much.