Why do some peoples teeth get bad again after they have their braces removed?

Rebound: Teeth have "memory" and if proper retention is not used then they can move. Retention all the time initially and at night after, forever is the only way to prevent rebound. Also different forces in your mouth can move teeth, so your ortho and dentist need to ensure that you have a good alignment and relationship of your teeth and bite and habits.

Relapse: In the process of moving your teeth through bone and gingival tissue, elastic fibers are stretched. These fibers tend to try to constrict thereby pulling the teeth back to thier former positions. Wearing the retainers properly and long enough should minimize relapse. Then again, we all change over time, so that has something to do with it, too.

Retention essential: Once teeth are moved orthodontically, they need to be held in place or they will tend to revert back to their original position. An orthodontist will discuss this with patients at the end of treatment and make them some sort of retainers, either removable or fixed or both. If you don't follow their instructions, your teeth will move over time.