Here are just some facts and one rumor that will explain everything, most just from articles I have read and postal workers I have talked too.
Last summer the new PMG ordered a bunch of processing machines to be removed and sold. We are talking specialized, multiple million dollar, MASSIVE machines. Some sold on eBay for $4000. A Judge ordered it stopped until at least the election was over, no idea if it restarted.
UPS and FedEx, by their own announcements, couldn't handle the package volume over the holidays and just refused to pick up some shipments from some retailers, I think Macy's was one? So, guess which of the big three didn't refuse packages! I know FedEx has the WalMart contract and there were local reports from post office carriers walking in their post office one day to a MOUNTAIN of walmart packages everywhere. They usually had one or two per route per day, that one day they had dozens each. We are talking a freak thosands percent increase that came from no where and left just as fast, on top of normal packages. My guess is one day FedEx said no for that area. And all of these are had to fit them in their blazer size, 33 year old, Iron Duke powered rattle death traps that catch on fire regularly. One day, the whole city was running late, the supervisors had asked the Amazon truck driver why they had 3x the normal Amazon packages. His answer? "Practically No one showed up for work at prime delivery so they redirected them to you".
Some areas of the USPS were completely overwhelmed beyond belief. there is a drone photo from Ohio where a facility is circled with semi trailers because they couldn't get through it as fast as it came in.(the rumor:) I heard personally from a letter carrier here in OK they got word L.A. was so backed up the PO there was looking into flying mail carriers in from around the country to help out for a short time