Concrete settles for a variety of reasons. One of the most common reasons result from the excavation of a basement.
When a basement is excavated, a larger area than the actual basement size is dug out. Excavation is done to give the workers room to build the basement walls. After the walls are up, you are left with a hole around the basement walls. Workers first fill it with one foot of stone on the bottom (on top of the drain tile) and then backfill the rest of it with the same ground that they previously dug out. This ground isn't compacted, however, and tends to settle in the years to come, taking sidewalks, driveways, and patios down with it. This is why you will tend to see most settling occur near a building.