system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do my teeth start moving back to their original position when I don't wear my retainer for a few nights (after 15 years of wearing it)?

I got my braces off in 1998. I'm 27. My teeth still move when I don't wear my retainer for a night or two. Am I going to be wearing this retainer (at night) for the rest of my life? Why do these teeth want to move?
gpt
You might have a tongue thrust - i.e. when you swallow your tongue hits your teeth instead of resting on the roof of your mouth. Since you swallow so many times in a day, a tongue thrust can move your teeth subtly. 

It's pretty easy to figure out if you have a tongue thrust, just swallow normally and pay attention to where your tongue goes.