You are absolutely thinking the right things. Concrete slabs can be susceptible to settlement from a wide variety of factors. In some areas of the United States, naturally occurring soils can consolidate over time, including areas ranging from Texas up through to Wisconsin. Soil erosion also contributes to concrete settlement, as this is very common for locations with improper drainage. Concrete slabs built upon filled-in land can excessively settle as well. This is common for homes with basement levels, as the fill on the outside of the foundation frequently is not "compacted" properly. In some cases, the slabs pitch down, directing water down towards the basement level. Tree roots can actually heave a slab upwards; this is quite common along public roadways, especially within Metropolitan areas.