How and why does a swimming pool settle? What can cause this problem?

There is usually one primary cause of settling and that is poor installation. You dig a hole in soil and disturb it and then put in 4-5 tons of concrete followed by another 10 tons of water, that's a lot of weight. If the soil was disturbed too much and not back filled with proper compaction the ground will settle. How long was the pool in place.

Soil under the pool was not compacted properly or spring water has softened the soil and the weight of the pool and water is causing the pool to sink. Tough fix. How old is the pool?