During construction of a new home, layers of soil are commonly moved around or spread out to get to the desired grade level. When the home is built, footings may be deepened to extend below weak fill soils and avoid a foundation settlement issue.
The slab, however, remains on the fill soils. If the fill was poorly compacted, the fill soil compresses, settles, and a void is formed under the slab. In time, the slab cracks, breaks, and settles into the void.