It's not one thing and there are more than a few big slices in the blame pie. But my pick for the biggest slice would be Kam's holdout.
If all else had been the same, but Kam hadn't missed the first two games with his no-win holdout, Seattle is 6-4 right now (with an outside shot at 7-3), they are still alive in the division, and tied for the final wildcard spot. Kam did punch that ball out vs. Detroit which was enough to forgive him, though he probably would have done that without the holdout too.
Additionally, I think the cloud of his holdout going back to the preseason put things in motion for the mental toughness and communication issues in the secondary that have remained problems all season. I don't blame Kam 100% for ALL of these problems- Sherman, Williams, Richard, and PC/JS deserve some raspberries as well- but I do think the biggest problem of the 2015 season is no longer the OL but the inability of the secondary to close out games, and I think Kam's holdout caused the LOB to lose its mental edge.
I know it sounds like sports talk drivel, but cohesiveness and chemistry do matter in football. When the team lost its way in 2014 it was none other than Kam who got the team back to playing for each other. So when you take the team's most vital unselfish player and turn him into the most destructively selfish player, it's going to reverberate for a while.