When your building structure was erected, the contractor likely stirred up the soil surrounding the building site resulting in poorly compressed soil. And even when it is compacted well, it’s never as stable as the original soil. Eventually, this may result in the slab settling and voids to form underneath.
Another cause of settlement is drought. During a dry climate or even just during periods of warm weather and low precipitation, the soil under your concrete will dry out and contract, forming voids. The concrete sitting on this soil eventually cracks and sinks into these empty spaces.
When the rains return, the water can easily get under the slab due to cracks and gaps left over from the dry period. The wet, soft soil is just too weak to support the concrete above it. In the worst case, the soil wears down and washes away completely, leaving behind large voids that cannot support the weight of the concrete above.