Sounds like ground water between the liner and frame. I have a concrete shell with a liner and had a similar issue. We had a burst of rain, about 5" in an hour and water pooled on my deck. Then suddenly it was gone and a bit later we had a bulge in the liner. What had happened was the water couldn't drain in the saturated soil so it pressed up against the pool. Normally this isn't an issue because the weight of the pool water keeps the shell sealed. But the hydrostatic pressure was so high that the excess water forced its way between the liner and shell.
In my case I pumped out behind the liner. There was a bubble in the deep end as well. I pumped as much out as I could then made sure the liner was set in the corners and along the wall. Overnight as the ground water receded the liner moved back into place. The weight of the water in the pool pushed out the ground water through the same little cracks it came through.
The question is how do you fix it? You need to relieve the hydrostatic pressure behind the pool. I spoke to the PB who rehabbed our pool extensively about it. He said in our area he has 1-2 calls per year with this issue. Correcting drainage around the deck is the first step, this is what I need to do. I'm tearing up a small rock bed and going to put in corrugated piping that connects to my downspouts, which eventually dump into the street (we live on a hill, street is 1.5 stories below the living area of the house).
A second more costly fix is to dig a sump near the pool. A contractor will dig a hole that's deeper than the deep end of the pool, line the hole, fill the bottom with gravel and put a pump in there. Water will flow there instead of pushing against the pool and you can pump it away. Depending on your soil and deck this might be easier. I'd have to rip up concrete and a wall to do this.
Look at your yard, look to fix any areas where water might be draining towards the pool. Change the grade to drain away from the pool. Look for any pooling areas that could be pushing up against the pool walls. As a last resort you could always install a french drain around the edge of the pool. I would have thought they would have installed this when they built it.
