I'm on my fourth year of my current fortress. Autumn came, and with it both liaison and the caravan. Liaison goes on to have a meeting with my mayor and caravan heads straight to the depot. Everything is going normal so far, so I order some junk to be hauled for trading and go on to take care of other things.
Suddenly I get an announcement of a puppy being killed. Then another one. This happens around the same time my quarantined wereskinks transform, so I go looking if I accidentally managed to leave one loose. There are no wereskinks to be seen. Instead, I see caravan guards shooting at my animals, all the while their wagons keep on moving towards the depot. I order my military to station on the courtyard just in case there's trouble to be had, and sure enough they engage the merchants as soon as they see them. Wagons roll to the depot, then leave immediately, and caravan guards keep fighting my military until they die. The guards, that is. My weaponlords won't go down that that easily.
Meanwhile, the liaison elevates my fortress into a duchy, makes a trade agreement like usually, and leaves without any trouble.
What exactly could be the cause of this? I've had a couple of sieges whose remains haven't been completely cleaned out yet, so the area around the main entrance is littered with corpses. I also saw a werebeast corpse in my courtyard, but it belonged to my former militia commander who was slain a couple of months earlier, so I doubt that has anything to do with it. Other than that, there wasn't anything out of the ordinary going on. No one was tantruming, no invasions were going on and no hostile wildlife was anywhere to be seen. I'm using the latest version of the game, in case that's any help.