Here's the scoop on this. The handstamps that are used to cancel the stamps at local Postal Outlets have the name and address of the Postal Outlet on the handstamp itself. When it is used for cancelling the stamp and the letter sent to the sorting warehouses, the letter is mistakenly sorted using the address of the postal outlet on the handstamp. The it is sent back to the Postal Outlet.
I had this happen several times years ago when the new handstamps were introduced and had to remail my mail. Now they don't poatmark the mail and my mail gets sent out.
Here's an example of the postmark.
