When it happened to me My best guess was syphoning....
Depending on the angle of the tank if the water level allows for enough suction to be generated against the overflow valve, then it would drain in the same fashion a fish tan k is drained.
suction is generated, and then as long as the water remains over the end of the tube, and the opposite end remains lower than the water level. then it will continue to overflow.
I added a hose end and a ball valve to be able to prevent this from happening again during travel.
2016 Jayco Jay Series Hardwall 12BSB
2004 Jeep Liberty Renegade