When I lived in Florida my neighbor had a company come out to work on their pool. Before the company drained it, they put in shallow wells all around the pool and pumped the ground water out. I asked one of the workmen why they did that and he told me that the pool would pop out of the ground when they drained it if they didn't pump the ground water out.
They had to keep pumping the whole time the pool was empty to keep the water table down around the pool. So the problem of pools popping out is well known in Florida (which has a very high water table). The owners of that house must not have known about it.
Mike
[Florida has lots of heavy rain. All year long, as I remember it. But it must have been seasonal.]