Foundation settlement can occur from a variety of conditions:

Poor fill soils:  When fill soils are used to support foundations, they should conform to the engineering guidelines pertaining to structural fill (because the soil has to structurally support a building).  Geotechnical requirements for placing structural fill are contained in both Building Codes and ASTM standards.  These guidelines require the soil to have particular gradations and to be compacted to a minimum density when placed as structural fill.  When these soils do not meet the proper standards, they will compress under building loads, allowing the structure above to settle. 

Buried Organic Debris:  Tree stumps are commonly buried and then built upon some time later.  After about 20 years, wood rots to a point in which a void develops underground, allowing the soil above to settle in. 

Organic Peat:  Prior to today’s more stringent building guidelines, wetlands were reclaimed and developed upon.  It was common to fill in a wetland, occasionally a lake as well.  Typically wetlands contain organic peat, which is a very soft soil that eventually decays.  Buildings founded upon these soils will gradually settle over time. 

Changes in Water Drainage:   Moving water can erode soil away from a foundation.  In some cases when outside grades changed (fill brought in, parking lot repaved improperly, etc.), a preferential drainage path develops underneath a foundation leaving a large void.  This can also come from a broken water line or even removing gutters.