I had two first class mail items, simple birthday cards, returned to the senders in Pittsburgh. The reason stated on the cards was that it was a bad number (address). The senders took pictures of the mail and the return stamp, and I showed them to my mail carrier (she's been delivering our mail for over 10 years). The letters were obviously addressed correctly and readable. She looked at the pictures, and she could see they weren't properly initialed by PO people to be returned to sender (not sure why). She then surmised that the letters showed up late at the local PO, perhaps because of mishandling, after mail had been given to the carrier for delivery, and a supervisor/inspector was coming. She said if he/she saw first class postage sitting around for delivery the next day, there would be a real problem. She said if it was sent with a tracking number, a supervisor would have taken it to the carrier for subsequent delivery. For them, since it was first class mail with no tracking, they could simply say the number was bad and put them in the return bin.
Sorry, that's the best I can relate the story. She used a whole bunch of acronyms I didn't understand.
Perhaps this is simply an issue with the local PO, I don't know. However, I've had many problems with the USPS. That is why I never, ever, send anything of consequence via USPS.