Sometimes villagers ask you if they want you to have them to move out so you can bring in new ones (they disguise it like "I heard they do crazy parties over there or something over there, and I wanna try something new!"). Of course, you can keep them in your town if you want. This happens every month.
If they move out, they may sometimes visit your town again on Main Street and wander around. You can talk to them and they'll say something like "Oh hey! It's been a while since I've seen you! Anyways, life's great!"
Anyways, definitely not a glitch you're encountering! Hope this helped!