I've seen that on my trailer once. I overfilled the tank and water started running out the overflow tube. When I stopped filling, it continued to drain. At first, I figured "no problem." But it continued for quite some time. I used my air compressor to squirt a little air back up into the tube and then it stopped. My conclusion was that by overfilling, I had started a siphoning process, which the blast of air stopped.
A valve on the end would solve the problem, but if you ever forget to open it there could be repercussions - flooding inside your trailer, perhaps.
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)