Clay-rich soils are 'elastic'. They expand and contract with moisture content. As soils become more saturated with water, the clay expands and loses strength. This condition allows slabs to sink just like standing in wet mud. This can occur from heavy rains, melting snow or plumbing leaks.
Many homes are built on backfilled soils. If the soil is not compacted correctly backfill will slowly and unevenly compact, sometimes over a period of a year, allowing slabs to settle.
Improper drainage can cause soil instability by creating areas of saturated soils allowing for slabs to settle. Poor drainage can be typical to the area or as minor as a misplaced down spout.
Trees and large shrubs can consume up to 30 gallons of water a day. If located near concrete, the loss of water in the soil will make the soil contract and can cause slabs to settle