There can be multiple causes for concrete to settle and require levelling, such as:
Improper compaction: When there is poor compaction of the base, the slab’s weight will further compress the base after curing, in some cases, causing settling.
Soil: Concrete is prone to sinking where heavy clay soils are present, as they enlarge when wet and reduce when dry.
Trees: Trees absorb large amounts of water and can sometimes dry out the surrounding soil. This may cause neighbouring concrete to settle.
Climate: When there frost is in the ground, freezing and thawing expand the slabs. In many cases, when the ground thaws the concrete will not settle to where it was initially.
Abrasion: Damaged or poorly placed downspouts and sewer lines can cause wear and tear to materials under the concrete.
Traffic or Machinery: Frequent heavy traffic vibrations in industrial areas can also cause slabs to move or settle.