In other words, how do the brains of those with BPD differ from those who don't have BPD?
The Wikipedia article doesn't even contain any possible explanations whatsoever.
The Neurobiology and Genetics of Borderline Personality Disorder indicates that a good deal of research has been done but a specific mechanism causing it has not been pinned down. It appears to be largely genetic which would strongly suggest a neurobiological/nature basis as opposed to a "nurture" related cause. (emphasis mine)