I once asked my wife (a social worker) why she thought so many people that "go postal" are postal workers.

After a little reflection, it occurred to her that many attributes of the postal service probably tend to attract the less than stable - rigid rules, a semi-government organization and rules structure, pseudo-police uniforms. She speculated that many people that might have otherwise been interested in police work or the military might end up at the Post Office after failing to make it in, or even into, those professions.