If it is defective, and you have a fairly full black tank, and you corner at speed, it is possible for the water to slosh with enough pressure to backflow.
Hence, why it is mounted high above the tank and why it is an anti-siphon, one way valve.
Cash ACME brass 1/2" FNPT anti siphon vacuum breaker valve.
https://www.lowes.com/pd/Cash-Acme-B...ker/1000385647