Houses experience differential settling for a variety of reasons including…
Excess moisture in the soil. This can be caused by poor drainage around the foundation, or by plumbing leaks under the foundation.
Problems with tree roots. Tree roots are always searching for water and if they manage to slurp up all the moisture in the soil under a foundation, the soil will shrink, and destabilize the foundation.
Expansive soil. Expansive soils are those that swell when they absorb moisture and shrink when they dry out. In other words, the soil isn’t stable. It moves a lot. This movement over time will destabilize a foundation.
Bad construction. If the soil under the foundation wasn’t properly compacted prior to construction, it can cause differential settlement in the structure later on.