Poorly compacted soil: When your home was built, soil movement around the building site results in poor compaction. Even when the replaced soil is compacted well, it's never as stable as the original soil. Over time, this can cause voids to form and slabs to settle.
Dry soil: Another cause of soil failure is drought. During dry conditions or even just during periods of warm weather and low precipitation, the soil under your concrete will dry out and shrink, creating voids. The concrete above eventually cracks and sinks into these empty spaces, especially if any weight is placed on it.
Flooding: When the rains return, the water has an even easier pathway under the slab due to cracks and crevices left over from the dry period. And this wet, soft soil is too weak to support the concrete above it. In the worst case, the soil erodes and washes away completely, leaving behind large voids that cannot support the weight of the concrete above.