There are many reasons that concrete can settle such as poor drainage, poor compaction, poor site conditions and tree roots. The most common reason we see settling is due to washout, which is when water from gutters and rain wash dirt out from under your sidewalk or driveway. This creates pockets or voids underneath your concrete and allows it to settle. Did you know that if a sewer line breaks, the sewage can wash dirt away, forming miniature sink holes.
Another factor is due to the clay rich soils that expand and retract with moisture content. As soil becomes saturated with water, the clay expands and as the clay dries out it retracts. The constant change in the moisture content allows for unstable soil and for your concrete to move freely.