Good question. Although I don't have a full answer, here's an experience of mine. Someone decided to follow me, and left a piece of fan mail. Sometime later, that person unfollowed, and then 're-followed' a few days later. The fan mail appeared again, exactly as it was before.
My conjecture is that un-following causes fan mail to go into a dormant, invisible state. Re-following causes the fan mail to bloom again, like flowers after a good rain in the desert.