I believe that backup happened at multiple throughout the country, honestly. I had 2 packages, one to San Diego and one to Atlanta. Both have yet to arrive and both were mailed early December. I'm still getting monthly "we're looking but haven't found them yet" emails from them.

I read awhile back (but can't find the source anymore, so grin of salt) that when USPS processing centers get overwhelmed they switch from a queue based system to FIFO. Which would make sense that some packages have been lost for months while newer ones seem to be going through the system just fine.