Halton Home Inspector wrote:
It is hard to imagine that the water table around your home and pool is so high that it would displace the liner.
The water behind the liner is probably pool water that has leaked through holes in the liner itself. How old is the liner ?? Are you aware of any cuts in the liner ?? Often water leaks behind liners where there are fittings, like the jets or around the skimmer.
Even if you have a high water table, the weight of the pool water would hold the liner in place.
IMO you have a leaky liner.
I agree with H H inspector. You got a hole in your liner somewhere. The pool water should hold liner in place. I have a 36 x 18 in ground in clay. Halton Pools on Main St. the best. They can check for holes, rips in liner. Had my pool for 30 years.