The root causes of voids are often poor compaction of the fill materials, erosion, high water content (shrink and swell cycles), decaying organic materials, freeze and thaw cycles, animals, and vibration.
These voids, if left unattended, will lead to the eventual breaking of the concrete. Voids can lead to erosion by allowing water to travel under the slab. These voids can be very common under driveways, attached garages, and walkways.
In industry the vibrations of machinery cause the supporting base material to vibrate, which allows the finer particles to settle out and consolidate lower down, leaving a void; resulting in settled and eventually broken concrete.