theFUZZ

officer.com
Is this nuts?

Talk about workplace violence! Police work is filled with Type A personalities and alpha males, but typically work place violence is confined to some pushing and shoving, or maybe a fist fight. At a department I used to work for, I witnessed a fist fight and there was a stabbing years before I started, but nothing where another officer had to shoot a co-worker. 

If I had to offer speculation, the argument was probably over a woman, it usually is with cops.