There used to be always someting strange about Expedia showing NWA fares for TYS - VIE route (I do not know about any other route as I fly only this one). When I searched during the last year for fares the only that poped up was the one with routing via IAD with TYS-IAD on UA. I always had to use "Multiple cities" to force Expedia show NW/KL via MEM or DTW routing.