I'm not sure, exactly. I admit I have tested it for about a minute, and although my pump survived, it made disturbing noises. Definitely abnormal.
No part of the pump moves more than 1/8"....it is not a typical impeller design. If I understand correctly, during normal pump operation, there is a plastic plate that sits atop 3 adjacent chambers, and the plate rocks back and forth to create pumping action.
Here's a diagram.
Obviously, the pump creates water pressure. Perhaps when there is back pressure into the pump outlet, the cumulative pressure created by the pump and the back pressure from the shore connection is enough to dislodge or rip the seal. I don't know, I'm just guessing.
Dave