If the discharge line has a slope and the end is below the pool water level you could get into a siphon condition. I've got a 140 foot run slightly downhill and once just shut the pump off after backwwashing and forgot to throw the multiport valve. Drained the pool to the skimmer bottom overnight. At the filter discharge I put in a PVC Tee and a check valve open to air. The valve is in a direction to let air in and let the long pipe drain when I shut the pump off. When backwashing the check valve is blocked and water just goes down the waste line.
Al