There are many different factors affecting movement of a tooth and have your teeth move after braces. If your treatment with braces is incomplete, then one factor causing relapse is that the soft tissue fibers attached to that tooth will be under tension in certain directions. They will pull it back like an anchor tugging a drifting ship back to its immediately preceding position.
To really answer your question, it’s helpful to think about teeth and their movement differently. Teeth never move back to an ‘original’ position. At any moment in your life there is a position that they are moving toward. Your teeth in fact never stop moving. In fact, they all move so slowly that you just can’t see it happening. The position they are drifting into changes as time passes. People who’ve had braces and people who haven’t both notice their teeth moving by the time they start to tip outwards, inwards, or overlap. This is why people in middle age discover that braces are for them after years not minding the mild crowding.
Do teeth move after braces?
The teeth can move because they are not like a house built with foundations driven & cemented into the earth. They are floating gently on a cushion of soft tissue in a sea of bone that constantly changes shape. Around each tooth root is a sheath of fibres, a ligament, which works as a shock absorber. Pressures and tensions in that tissue are from all kinds of things including your bite force and direction, the pressure of your tongue outwards and your cheeks/lips inwards. These all cause a complex interaction between the cells of the soft tissue and the surrounding bone. They can dissolve away bone and regrow ligament in any given direction.
What do braces do?
Pushing, pulling or torqueing a tooth with braces can move them all around. But they do this on their own constantly anyway. The bone nearby is shapeshifting too. Your bones turn over – they disassemble and reassemble perpetually, remodeling, depending on the work you have them do. This is why astronauts on the ISS lose bone density under low-G conditions as their muscles do less work to move their bones around in space, and they need to do extra workouts to maintain bone density.
Listen to your orthodontist!
The ‘original’ position that you’re picturing was a position of all the teeth at which they each were feeling a similar amount of pressure in all different directions. We call this equilibrium position the ‘neutral zone’. External (cheek lips tongue bite) and internal (ligament fibres etc.) forces balance out to hold each tooth in its place over time. If you’ve shoved all your teeth into a more attractive or more useful position, in which the lips happen to pull in far more strongly than the tongue pushes out, for example, your front teeth will obviously want to drift back to tipping inward. Your orthodontic treatment outcome is never a long-term stable position. So listen to your orthodontist when they tell you retainers are needed for life!