Typically a swimming pool's age, is the main cause of Pool Leak problems.
Other causes are poor construction techniques, improper winterization of pipes, ground settlement that just breaks a pipe, liner gets brittle and loses its elasticity which can easily rip or get a hole. Skimmers start to pull away from Gunite Pools and cause Leakage too. Termites cause a lot of damage too --> whether its them eating thru the liner or a certain type of piping (Flex PVC) probably thousands of Long Island Pools have termite problems.
Most pools will eventually Leak. Common Issues include: