Freeze-thaw and moisture
Water that enters joints or cracks can expand when it freezes, stressing both the slab and the base.
Repeated cycles wear away fines and loosen support, especially if drainage sends water under the slab.
Soil and construction fill
Loosely placed fill or organic soils can compress for years after construction.
Utility trenches and landscape changes can redirect water in ways the original pour did not anticipate.