There are many reasons why concrete slabs sink. Some of those include:
Poor drainage – Poorly placed gutters can spill water too close to your home. When water constantly flows underneath your slab, the soil can wash out.
Poor compaction – Soil compaction is when dirt is packed together so that there are no spaces between the soil particles. If the soil laid underneath the slab was improperly compacted, the slab could sink.
Poor site/soil conditions – You don’t want the soil beneath your slab to be made of too much clay. That type of dirt expands when it gets wet and shrinks when it dries, causing the soil to shift.
Tree roots – Tree roots search for water in your soil. If water is soaking the soil beneath the slab, tree roots can start to invade, pushing or sinking the concrete.