...You had them attack EVERYTHING?
Yeah, probably some dwarven traders.
Here's how loyalty cascades work:
0. You have a fortress of, oh, 20 dwarves, 1-20. They are all members of the civilization (let's designate them C) and of your fortress (F). Let's throw in a merchant caravan, three dwarves (@,#,$), who are C but not F. When something attacks something else, it becomes an enemy of the target's groups; e.g. if Q attacks F (who goes to the hospital), Q becomes an enemy of the civ and the fortress. Let's designate such an entity as cf.
1. You order your militiadwarves--let's say H, E, and L--to attack everything. L attacks a member of the caravan. L is now an enemy of the civilization, but not an enemy of the fortress. Therefore, L is cF.
2. H and E notice L. L is, as an enemy of the civilization, their enemy. They attack L, killing him because L forgot to bring his helm and axe. This is another important part--E and H attacked a member of your fortress, making them enemies of the fortress, but as L revoked his citizenship when he became an enemy, they are not enemies of the civilization. E and H are Cf.
3. E and H wander into the alcohol stockpile. They see a bunch of enemies--e.g, other members of the fortress. A, M, T, J, and K counterstrike and become cF. Basically, everyone becomes either an enemy of the civilization, an enemy of the fortress, or a corpse. One of the two "factions" will eventually kill the other if they don't get separated or killed by some other means first.
4. Once you're down to one group of dwarves, all enemies of the same group, you begin to rebuild. With luck, you avoid a tantrum spiral.
5. Migrants arrive, enemies of your current fortress residents.
6. Go to step 3 until all dwarves who are enemies of your civilization or fortress are dead.