The reason for that was that the US government tried to guarantee jobs for veterans. The one government task that could use relatively uneducated but fit people no matter where they happened to live was postal delivery. So a lot of veterans with PTSD and weapons training wound up as postal delivery workers.
Veterans with PTSD and weapons training sometimes flip out and shoot people.
I recall efforts to reduce stress as apparently managers were tired of being murdered.
Maybe new phrase will be “Going Amazon”