Ok I put up a bug on the tracker,
http://www.bay12games.com/dwarves/mantisbt/view.php?id=3685, where I noticed goblin ambush squads arriving at my fortress would immediately kill each other at the edge of the map.
Going through Legend mode showed that a goblin faction, The Clean Malice, which is a group from the goblin civ The Dread of Trails, somehow became an enemy of its parent civ. The attacking goblin squads contained goblins (and humans) belonging to the Clean Malice, and non-Malice goblins were killing them. Essentially, loyalty cascade but this time on the goblin side.
Digging deeper into legends, this is how I think I managed to produce this situation:
1. Mod your game so DEFAULT_CIV for goblins is CITY instead of dark fortresses, so goblins spawn with human-like towns.
2. Find a goblin civ and a town/hamlet founded by a subgroup of that civ. Ideally this town would be close to potential embark sites for your future fortresses, and invaders would be coming from this town.
3. Create an adventurer from a civ at war with the goblins, and have him get some reputation (enough to be able to hire four or five companions).
4. Go to the goblin town you picked with your adventurer (make sure you have free slots for hiring more companions) and attempt to get a random goblin to Join your group. This should be possible if your adventurer has not killed any goblins yet (and is unknown to the goblin civ).
5. Watch as all the other goblins pummel the hapless gobby you hired. Watch them kill each other afterward for murdering the goblin.
6. ? ? ?
7. Profit! *
*"Profit" in this case means goblin mayhem, and a relatively safe environment for fortresses in the area, as goblin ambushes will reveal themselves as soon as they enter the map, and goblin sieges will soon devolve into vicious free-for-alls.
It should be possible to produce similar human or elven loyalty cascades if you can spawn adventurers from civs at war with them.