Well, there are numerous reasons why this could happen, but the most likely is that the resurface job that took place 7 years prior was not prepared correctly. To save time and to still profit from an under-estimated job, the 'contractor'(probably unlicensed) either did not remove all the hollows in the old surface or simply did not use any bonding agent between the old and new surfaces. The homeowner probably received 2 or 3 quotes prior to making his choice of contractors and wouldn't you know, the cheapest contractor promised the same service as all the rest and was about $400 less. $400 is about the cost of the preparation stage prior to resurfacing which includes all bonding agents and labor to undercut and remove all hollows/pop-off areas.
In S. Florida during 2004 alone, more than 4700 homeowners reported these hollows and bubbles that had formed in their new pool surface. When asked to provide the copy of the contractors license and insurance that they were given at the time of the job, homeowners replied " They did not give us their paperwork"
They did not provide you with the paperwork or you did not ask for the paperwork? Big differnec!
Remember, If you live in Florida and you're hiring a Pool contractor, you need to see a License provided by the Dept of Business and Professional Regulation, plus a copy of the insurace showing Popping coverage. FYI, one pool per day in Florida pops at least 8 inches out of the ground. How do you repair a pool that has popped. You build another one.