When this game is loaded, you'll see that the outpost liason is headed off the map. When he exits, the fort will shortly become a barony afterwards. In the main entrance of the fort, there are 3 militia dwarves (can center on it by hitting F1). One is the mayor, who will become the baron. You need do nothing but observe the fort for a while.

A dwarven caravan is at the fort, and usually by the time it has departed (sometimes a little before, sometimes just after), the random killing of fortress citizens will have begun. The time may vary, but eventually the wrong person will pass by those training militia dwarves, and the killing begins, which I believe additionally leads to a general loyalty cascade.

This fort ran for 8 or 9 years with no issue like this at all. This problem begins only once the fort is promoted to a barony. I even reloaded an old save to before the barony declaration, and observed everthing running fine for an additional year, by refusing to allow the fort to be declared a barony. But, the next year, when allowing it to be declared, I got the same behavior. Eventually those soldiers catch sight of someone they don't like (maybe a spy, unbeknownst to the player?) and start killing citizens, and sometimes visitors.

If you would like to test the whole non-barony thing you could use DFhack (if it's working with 47.02) to kill or stop the liason from leaving somehow. If you do, and the fort remains a non-barony, you should see that peace continues to rule the day.

The game is unmodded, except that it is running the Ironhand tileset and cats have had the ADOPTS_OWNER tag removed from them.