I just had a new liner installed. The evening after the installation we turned on the light inside the pool and see divots everywhere. One section near the deep end there appears to be a slide of the sand underneath the liner causing a huge sunken area in the wall about 3-4 ft down with a large mount below it. The installation company said it was all caused by ground water from a brief rain shower. This being something that wasn't covered by his warranty. Is this something that can actually happen?