There are many reasons concrete can settle, such as poor drainage, poor compaction, poor site conditions, and tree roots. The most common reason we see concrete settling is when water from gutters and rain washes 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 the sewage can wash dirt away if a sewer line breaks, forming small sinkholes? Another factor is clay-rich soil that expands and retracts according to moisture content.