A lot of flat land areas can do this as well as valley floors. In the OP's case it wasn't just water "behind the liner" . It was, or could be, acres of area that need to be drained from around the pool. He only really needs to get the water below the level of the water in the pool to keep the liner from floating.
We have several gravel pits in my area that have standing water in them from the water table level. One guy who was not familiar with the area bought one of them and thought he could pump it out to continue mining. Had a large volume pump running for several years but never made a dent in it. The level would fluctuate throughout the year by about 10 feet but never much more than that. Fortunately for us, the level stays about 30-35 feet below the level of the land in the flat areas. Makes it nice if you want to hand drive a well for irrigation though.