Its unusual to see it happen, but if the weather conditions were so bad that the item couldnt be delivered safely and x number of attempts were made it is returned to sender.
If the item needed a signature and the recipient was nit at home, they would be left a card. If they didnt ever go to collect it the item would be returned to sender.
If the house is boarded up, or the letterbox inaccessible the item would be returned, or if there's an entry system such as those found in a gated community or a block of flats, and nobody answered, the same would happen.
I'm not sure if any of these situations are relevant but they might help explain the type of circumstances that might have occured.