Too many packages into a system that couldn’t handle the load. It’s like blaming the water company because your toilet backed up.
I know several UPS over-the-road drivers and a few delivery drivers. Amazon dumped tons of orders on UPS over the weekend and Monday morning. Many of those packages were for the PRIME accounts.
UPS had extra people to try to handle it but too many drivers ran out of their Federally mandated hours and were legally not allowed to drive. That is where much of this fiasco occurred.