There are lots of causes for these cracks. Here are some of them:
Heavy clay content in the soil. If the pool was dug during a drought, the clay will have shrunk. When the drought broke, the clay absorbed moisture and swelled, creating pressure in the opposite direction the shell was meant to hold.
Pool build on fill. The fill settles and the pool loses support.
Pool built on a slope without special engineering proper done. The ground slides away and the shell loses support.
Insufficient water in the gunite mix used to form the shell or if shotcrete was used, the mix was mixed too long ago. These usually result in a crumbly mix and it's caught early, usually before plaster. Sometimes rewetted trimmings and rebounded material gets used too and creates weak spots.
If you are the original owners of these pools, notify the builder. Most builders offer a lifetime warranty on the shell.
Check to see if the water line is even across the tile line.
You may need a soils engineer's report to tell you about the supporting soil conditions and how to stop the condition from causing further damage.
Scott
