What would cause water to keep getting behind an inground pool liner?

Rain. It's normally just ground water that takes a while to drain away.

You may also have a hole you haven't detected. It happened to me.

I will agree to the leak answer but not the rain. There are a few scenario's to think about. If you have an inground pool and your pulling the liner out of the track and you see water actually behind the liner, and im not talking just because the wall is wet, im saying if there is actually standing water behind the liner, you have a leak, and most likely its a hole or holes in your liner. Now if you feel water under the liner on the floor, which should only be in the deep end, then you could either have a hole or holes in the liner, and the water hasn't soaked into the ground which if you have a concrete bottom then most likely its not gonna soak up anywhere unless it makes its way torwards the pool walls. And besides if you have an inground that is FULL of water, its impossible for any ground water or water from leaks to stand behind your liner. There is more pressure from a full pool of water then what is being pushed up from ground water, therefore your pool water will push all that water in the ground and you wont feel a thing. The ONLY way your gonna know if you have high water is either A: you know for a fact that you do or B: you start to drain your water out of the pool and you see the liner starting to float on the deep end floor. But you shouldn't be draining your pool completely out if you have liner anyways. The water is whats holding that liner in place and if you drain it out, your risking that it moves, and you cant get it back into place and you end up with wrinkles everywhere or worst case it rips by chance. Any company that installs liners uses special vacs to hold liners in place while its filling and stretching to the shape of the pool. Now your WORST nightmare of draining your pool out is if you do have high water and you drain it out, your risking the high water breaking the bottom of your pool floor and even the slopes ultimately leading to your pool quite possibly caving in. I've worked on pools and installed pools for 10 years and trust me I've seen it happen. But the best answer to your question would most likely be you have a hole or holes that is causing it. Simple way to find out but is time consuming is get some colored die, doesnt matter which, and dive around the area where you know you see the water behind the liner and die test the liner and see if any of the die goes through any holes. You found the problem then.

There are pipes running next to the pool. Some of which pull the water to the pump for filtration or vacuuming . Others bring the water back to the pool through the jets. There could be cracks in the pipes causing the problem. Around my pool there were two pipes that were cracked and the water made its way behind the line. It was obvious that the pipes were broken for a long time because the water had created a underground cavity right next to the pool. So that is something else you can look into.