I imagine the USPS uses rental trucks from time to time to move excess mail. Someone probably stole the truck and tossed the cargo when they realized it was US mail.
Thieves would rifle the mail and packages for anything of value. Dumping unopened mail is usually done by carriers.
USPS knows exactly which carrier was supposed to be delivering it.
that, or maybe the postman visited his girl friend for a few hours and didn’t have time to deliver everything, so, he discards it. Possible?