First of all there is someone here on hub pages that I strongly recommend. Not sure if I am allowed to disclose this here and now... Though if you look up bpd - you will find it listed...
To answer your question Bpd is a very controversial subject. There is a lot of info as well as much stigma surrounding bpd.
It is however, thought to be an illness regarding upbringing & traumatic experiences, though new medical facts also prove that it is a 'biological' condition and therefore is genetic and is hereditary amongst families.
Though also take note that in the psych- trials; Not all people were to do with traumatic cases in terms of upbringing. Statistic results showed that there were a number of borderlines who had been brought up in a normal & well adjusted environment.
Scientific reasons for this determines that the cause is unknown and that the major factor is to do with the genes. Being predisposed 'genetically' will increase the chances of bpd.
Updates of Bpd is that it is a 'biological' disorder. Controversial to the earlier diagnostic manual from the last several decades that has now been updated, it is now medically termed as a 'biological' medical disorder.
References seek google; bpd central; & many other info centres regarding bpd...
Interesting observations on bpd is that they are highly intelligent, highly sensitive as well as emitting very strong empathic skills when dealing with other people.