When a disgruntled, highly mentally unstable, or just plain wierd office worker goes into his place of employment and starts shooting up the office, usually with a semi-automatic weapon. This used to be very common amogst postal workers. Some people have actually done this at an ex-lover's workplace after a bad breakup. Most causes involve a series of frustrating events that go on for years and then one horrible event or day sets some poor motherfuckeroff the deep end.