I think "going postal" has something to do with doing a boring, mundane job for years and years on end. Jobs like these should have a high turnover, and would if these jobs were low-wage like they should be.
But union wages and/or government benefits are hard to give up, so people stay at these dead-end jobs for many years. Some of the unionized grocery store cashiers here have been on the job for 30+ years!!! That's enough for anyone to go crazy. Counting money...sorting mail...do it for a year to get on your feet then move on!
Newman (Seinfeld) on why postal workers go postal:
"Because the mail never stops. Everyday it piles up more and more and more! And you gotta get it out but the more you get out the more it comes in. Then the bar code reader breaks and it's PUBLISHER'S CLEARING HOUSE DAY!"