The Seattle Seahawks run defense was poor last season. They allowed 4.9 yards per carry (tied for 26th), gave up the sixth most first-down carries, and tied for the fifth=most rushing touchdowns allowed with 21. In seven of their regular season games they allowed 161 or more yards on the ground.
Despite this being a pass-heavy league, allowing rushing numbers like that is going to cost teams football games, something that was apparent against the Carolina Panthers and the Thursday night game against the San Francisco 49ers, both of which were played in Seattle.