Over time, rain water periodically saturates the ground
causing the soil and sand to become semi-suspended (mud). This allows the sand and soil to slowly shuffle and organise efficiently (with less space between each particle) as the water drains away. As the material below your slab becomes more organised through repeated instances of this process, it takes up less space below your concrete slab or footing which allows a void to form. This is what causes sunken slabs and cracks in buildings.