Concrete can settle for a few of reasons. (1) If the compaction of the base is done poorly the weight of the slab will compress it further causing the concrete to settle. (2) Climate change. Freezing causes the slab to expand and then when it thaws the slab will contract causing the concrete to settle. Heat and drought will cause the base soils to shrink, causing the slabs to settle and then when it receive rain it'll expand once again causing the slab to shift and become uneven. (3) Erosion. Improper drainage can cause the base soils under the concrete to wash away causing the slabs to settle. (4) Traffic. The vibrations from traffic can cause the base soils to compact which will lead to settling.