Some soils are not capable of supporting the weight or bearing pressure exerted by a building's foundation.
Excessive Moisture Content
Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts. The reduced ability of the soil to support the load results in foundation movement and/or cracking. The moisture is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table.
Expansive soils will swell when wet and contract when dry.
Lateral Sliding/Soil Creep
Soil creep is caused by slow downward movement of expansive soils under the influence of gravity and the effect of moisture changes. The depth to which this occurs varies depends on moisture amounts and soil types but can often times extend down below the depth of most shallow foundations.
Poor Fill Compaction
In some cases lots are created by adding soils brought in from off-site locations or cutting one area of the hillside and filling another to create a more level lot and when fill soils are not adequately compacted, they can compress under a foundation load.
Sometimes footers are either nonexistent, designed too small for the current loads or not built to the design at the time and hence have inadequate bearing capacity.
Consolidation occurs when the weight of a structure or newly-placed fill soils compress lower, weak clay based soils. Consolidation results in downward movement or settlement of overlying structures. Settlement caused by consolidation of foundation soils may take weeks, months, or years to be considered "complete."
Vibration from seismic activity.
Impact from Trees
As trees mature, their demand for water also grows and the root systems continually expand and can draw moisture from the soil beneath the foundation. Clay-rich soils shrink as they lose moisture, resulting in settlement of overlying structures. Foundations closer to the surface are more often affected by soil dehydration due to tree roots than are deep, basement level foundations.
Most likely causes include poor surface drainage, faulty drains, leaking water mains or other underground water movements.
Apparent Settlement Due to Construction
This is where the area in question was built with a slope to it, greater than normal such that it looks as if it has settled when in fact it may not have moved.
Concrete Spalling From Rusting Rebar
Rusting of the steel reinforcement in the concrete causes the steel to expand such that it is a strong enough force to cause the concrete to crack.