they say if a villager moves out and nobody adopts them from you then they are "in the void" which i guess is stored in the game's cache. if you then visit somebody, receive a visitor, or random streetpass, the other person can receive the voided visitor as a move-in for their town if they have openings. they say that your void clears after this, even if you visit somebody with no open villager slots. (i'm not sure if you can have multiple villagers "in the void" and if so, whether they all clear out or not after a streetpass). i think there are threads out there explaining all the variables involved.
the void is when a villager does not find a home. You can clean your void if you visit a town that have 10 or less (someone might get your voided villager if you visit a town with less than 10 villagers)