This happens mainly because of heredity, wisdom teeth (the growth can make teeth overlap in the front of the mouth), or even teeth that are lost prematurely due to extractions. This causes malposition of adjoining teeth and shifts their positions, which has the same effect as removing a block out of an arch: over time the entire arch collapses. Changes in how your tongue rests against your teeth and in your bite can also cause your teeth to shift.
Orthodontics, Invisalign, cosmetic dentistry and bridge work can help correct shifting teeth. It's important to address shifting teeth, since extra spaces can form where food particles can get caught and cause tooth decay and gum disease.
If your teeth have shifted and are also chipped and discolored, composite resin bonding, veneers or crowns over the damaged teeth can make a significant difference.