Band-aid 'underwear'- given by yellow haired guy in the brothel. Trigger is probably pigtails (has a scene with the undies guy)
Black string bikini 'underwear' - given by yellow haired guy in the brothel. Trigger is probably braids. (has scene with undies guy, counts as underwear, so no walking around in it (and yes I tried on the beach))
Bunny Mage armor- Craftable, borderline SFW when damaged. I believe the trigger is connected to the tower scene with Melissa. I lost. (modified beach scene confirmed with damaged armor, no unique ones found yet)
White bra+panties- Not actually glitched, is worn with bunny mage and cosplay armors. (no scene with undies guy)
3 cosplay armors- bandit houses, after tower is cleared solo. (modified beach scenes confirmed, no unique ones found yet)
Pink/Purp Dress- Eat at the maid cafe a lot, talk to the waitress you ordered from afterwards. Eventually, she'll ask you to give her 30 eggs, and gives you the recipe for this as a reward. (wearing schoolgirl+pigtails when triggered, but doubt that mattered, they key seemed to be not having the sweat-drop reaction after eating, could just be a random chance for each item each time you eat there)
Pink silky underwear- go to the spa type place (next to the pet shop) with the pink/purp dress + braids (string bikini was also worn, and could potentially be a trigger, but guessing it's dress + braids since it matches the dress, and they wouldn't let her in with the dress + pigtails, although she was in diff undies then)(also slightly possible I got this with the dress and just missed it, and that the spa was something else entirely)(undies guy scene confirmed)
Hooker outfit + Hooker underwear- gotten from the brothel bouncer. Kinda looks like a bunny girl with a lab coat to me... (reception girl wouldn't let her work with 60ish stam), not sure on trigger (possibly number of times worked there, possibly having worked there in a certain number of outfits, possibly had sex a certain number of times, possibly gotten both items from the blond guy, possibly something else)(undies guy scene confirmed)
Third hairstyle (braids) is also bought at the convenience store at night. Likely triggers: working there several times during the day or wearing the pink/purple dress (pigtails possibly required)
Melissa maid scene- talk to the noble in the sushi bar, pay him 50,000 (or maybe get payed? I don't really keep track of how much gold I have, it's too easy to get). Go home. Trigger is probably tower related.
Home rape(?) scene- not sure how I triggered this, did 3 fishing related things that day though. Fished at the world map spot south of the east bandit house until I got the achievement. Eventually it made me stop (with stamina left), I talked to the fishing guy and he said some words (instead of just giving me the fish shop). I was able to fish again afterwards. Then I went back to town, and sold some of every fish to the chef at the sushi bar (who reacted to me selling the most expensive fish). Pretty sure it was talking to the fishing guy that triggered it, but can't say for sure. 10k found in the bathroom afterwards. Also possible this was connected to having seen the beach scene with all 5 armor types, or not going to the brothel for a bit. Kinda looked like one of the beach guys, but difficult to say with sprites...
Hotel guy quest line: Fairly early on he had a [!] over him, initially I thought that was a trigger for undies guy, but don't think so anymore. Pretty sure he was requesting me to make him an item (one of the non-wearable clothing items on crafting choice 3), gave that to him a while ago, and recently he had another [!] over him, and it gave me what looked like the recipe received message. Not seeing anything new for any of the wearable options, but it's possible that he gave me a new non-wearable option, or gave me a cooking recipe (I don't pay much attention to those). Update: He's now a standard buying NPC, asking for 3 types of items.
The Bunny Mage and 3 cosplay armors show the same graphics when damaged even if you take her underwear off and she freaks out (visible damaged underwear shown, but you -can- take the armor off and only the underwear will be shown(no damage), but she freaks out if you leave the menu without the armor). Based off that, I'm guessing these are not true layered images, and she's not going to have the option to go pantyless, at least in armor.
Suspicions:
I'm guessing that the sheep themed outfit shown in the preview image is gotten from your neighbor. Possible triggers are cleaning up the animal pen a certain number of times, or getting a certain score. Considering this comes from japan, I'm guessing it's score. I really don't want to test this...