Finally got a siege in my new fort. My military rushed to fight it, but partway through I noticed a report of an axelord attacking my militia commander. I *know* the goblins didn't arrive with an axelord, so I take a closer look, and sure enough, some of my troops are attacking others of my troops, while in the middle of a battle with goblins. I took a close look at the axelord and the commander, and both were still citizens of the Big Sling (my dwarf civ), and both still members of The Blunt Boots (my group). So it doesn't appear to have been a "canonical" loyalty cascade. The battle continued and eventually no more "friendly fire" incidents. I ended up with the captains of both of my melee squads dead, plus 3 others. My best guess is that the squad captains were a couple of jerks and the troops decided to frag them at the first opportunity in combat.
Also saw a couple of interesting thoughts during the battle.
Urdim Bomreksakrith, Hammer lord: Sodel Stokedfortress is dead. I guess it could be considered grimly satisfying.
Urdim Bomreksakrith, Hammer Lord: Urdim Whippedblazed, slayer of Sodel Stokedfortress. There is no need for celebration.
Sodel Stokedfortress was the militia commander, who Urdim had just killed.
Urdim Bomreksakrith is Urdim Whippedblazed.