Maybe it relates to the mechanic that shows you, in adventure mode, which body parts are currently the easiest/hardest targets? Body armor adds difficulty to hit on the body parts it covers, and the ai is choosing the easiest targets?
Now ive had some bizarre behavior recently too, and i cant find a reason for it;
I made these neat war animals, like a giant crocodile with four grasping forelimbs, sextudiles (sextuple-crocodiles, for six legs, get it? Sextudiles) Now when i generated the world they were war animals, but not a large_predator or prone_to_rage, noemotion, nofear killers, which i added when i saw the adult, war-trained animals running away from intruders instead of attacking them.
Now theyre aggressive and go after intruders like theyre supposed to. However, when i penned all the hatchlings in into my depot enclosure and let a couple goblins in, they went totally nuts. Not just on the goblins, but each-other. The goblins got torn to shreds and only did meager damage to a few, but the sextudile hatchlings continued to tear each-other to pieces. In the end 10 or so were dead and i butchered a few more that lost the ability to stand.
Later i had a werebeast visitor transform in the middle of the tavern. Not just that werebeast but every visitor in the tavern at the time went mental and started attacking dwarves (which is anomolous but id chalk it up to the visitors being from the same civ as the werebeast and siding with it when dwarves recognized it as a threat, somehow). Dwarves fled the tavern into the sextudile pen, visiting mercenaries followed, and the sextudiles again attacked the mercenaries and then one-another. These were war-trainwd adults, the parents of the crazed hatchlings.
I still have no idea why theyre doing this. It doesnt make sense.