What Causes Concrete to Settle

Poor or improper compaction of the base before concrete was poured: The weight of the slab will further compact the base after curing, and sometimes settlement can happen quickly.
Soil types: Where heavy clay soils exist concrete is prone to heaving and sinking, expanding when wet and contracting when dry.  
Tree Roots: A large tree can consume as much as 400 litres of water a day, drying out the ground around it and causing nearby concrete to settle. If your driveway or sidewalk is close to a large tree, this may be causing your concrete to settle.
Climate: Freezing and thawing cause slabs to expand when frost is in the ground. 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 and excessive rain can all lead to a washout of base materials under concrete causing the slabs to settle. Remember polyurethane foam will not erode like traditional mudjacking.
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.