Going crazy. Flipping out. Losing it. Going over the edge. Becoming psychotic.
Refers to a not-long-ago spate of US Postal Workers going in to work and shooting up the place. Guess they didn't have a decent grievance process.
According to the USPS website, there are 800,000 postal workers. That makes them one of the biggest (if not the biggest) employer in the nation. Even if postal workers were less likely to freak out than the rest of us, I suspect there still be (in absolute terms) more berserkers in the Post Office than anywhere else.
Actually, the Postal Services has a rate of workplace violence that is a little on the low side. Unfortunately for its reputation, there were two or three incidents close together that gave rise to its violent reputation.
There was a rather violent video game a few years back called "Postal", in which you were a nut with a gun (not to mention lots of other, more powerful armaments that, in the grand videogame tradition, just happened to be lying around), trying to kill off as many cops and bystanders as you could before getting killed yourself. In the final level, you are going "postal" in an Army base.
Many countries banned this game.
Also a DiscWorld novel by TerryPratchett.