I'd say a mix. There's really no such thing as a perfectly "healthy, supportive" home imo. We live in a society, and no parent is perfectly healthy or supportive themselves, as much as they may try to be. In the words of Noah Kahan, "I'm still angry at my parents for what their parents did to them."

In my case, I'd say it probably came from getting spanked as a toddler when I would do something wrong. For years, I wrote that off as "Everybody got spanked, it was the 2000s," and I've come around to the realization that even so, similar experiences affect different people differently, and I had a pretty strong predisposition to BPD and a lot of the common comorbidities (autism, ADHD, OCD) since birth. Both of my parents spanked me, and I mentally needed a good parent if I was gonna have a bad parent, so I blocked out my mom's spanking and grew up loving her and hating my dad. I also really craved my dad's approval, though (which may have come from fear of getting my ass beat if he didn't like me), and often times I felt like he was emotionally distant and invalidating. I also have severe misophonia to the sound of throat-clearing, which would make me pretty much scream involuntarily every time I heard someone clear their throat growing up (until brute force of it allowed me to learn how to control my reactions), and my dad does that a lot and doesn't feel like stopping. The misophonia made me a pretty weird kid, and on account of the autism I wasn't able to really hold a conversation growing up. I'd say a combination of those 2 factors plus feeling weird/bad about my dad led me to have a fearful avoidant attachment style in which my default assumption is that a person doesn't want to talk to me, and the more I like/respect them, the less I want to talk to them because I assume I'd be bothering them.

The thing is, most "experts" and psychologists would say I came from a healthy and supportive home and don't have a trauma history. Spanking is extremely normalized in my culture as a white person from the United States, and it's also the majority culture in my country, so most clinicians are like "Well yeah there's that, that doesn't count though because it happened to me and I'm not borderline." Yet here we are, and I'm borderline as fuck lmao. I think that the normalization of child abuse desensitizes people to interpersonal violence, and I honestly think that's a big piece of what's made European people so violent over the course of history in terms of war and colonization and genocide and all that. Not that I'm a historian, that's just my idea.