Poor or improper compaction of the base before concrete was poured: The weight of the slab will further compact the base after curing, and settlement can happen quickly.
Climate: Freezing and thawing causes slabs to expand when frost is in the ground – it causes concrete to heave or raise. When the ground thaws, the concrete will settle again, but many times not to where it was originally. This results in trip spots between shifting slabs. On the opposite end of the climate spectrum, heat and drought can cause expansive clay soils to shrink, causing the slabs to settle. When the clay soils receive rain, and expand once again, the concrete slabs shift and become uneven.
Erosion: Damaged water and sewer lines, improperly placed downspouts, excessive rain, and compromised retaining/sea walls can all lead to a washout of base materials under concrete causing the ground to settle.
Machine/Traffic Vibrations: Concrete slabs may shift or settle on highways or in industrial settings where there is frequent heavy vibrations or loads being transported. The vibrations from machinery and passing traffic can lead to the base compacting and slabs settling or moving.
Slab Curl/Rocking Slabs: Slab curl occurs when a relatively large section of concrete is poured. During the curing process, the top slab may cure slightly faster causing the slab to curl, rock, and become unstable.