Was this a single fare component or a broken fare? If you purchased travel from SEA to ORD and then travel from ORD to SMF, then the airline is obligated to route you through ORD (even if you would prefer a different routing or a nonstop if available). OTOH if you purchased a fare from SEA to SMF with no stopovers, then the carrier is simply required to fly you from SEA to SMF by any routing that doesn't significantly impact your original departure and arrival times.