If, for example, you move from one state to another, your previous state may not know you don’t live there anymore. So if they see you haven’t voted in that state for a while, they’ll assume you don’t live there and remove your name from their registration.
They always know, especially if you’re paying your taxes correctly. Try moving to Colorado. You’ll get registered immediately without doing anything and your ballot will be mailed to you. You don’t ever have to do anything in person.
There’s no reason to unregister someone from voting. In any state.