My understanding with BPD is that it's triggered by an extreme fear of abandonment due to some form of trauma/neglect in the past.
http://www.nimh.nih.gov/health/publi...disorder.shtml
<snip>While a person with depression or bipolar disorder typically endures the same mood for weeks, a person with BPD may experience intense bouts of anger, depression, and anxiety that may last only hours, or at most a day.5 These may be associated with episodes of impulsive aggression, self-injury, and drug or alcohol abuse.<snip>
Borderlines tend to run co-current with any combination of other personality disorders..
http://www.nami.org/Template.cfm?Sec...ontentID=44780
<snip>Co-morbidity with other disorders:

Major Depressive Disorder -- 60%

Dysthymia (chronic, moderate to mild depression) -- 70%

Eating Disorders -- 25%

Substance Abuse -- 35%

Bipolar Disorder -- 15%

Antisocial Personality Disorder -- 25%

Narcissistic Personality Disorder -- 25%<snip>

The reason, imho, that it doesn't 'appear to be a personality disorder' at all is because it is very difficult to diagnose. It's very difficult to find a therapist that will work on this....and it's very difficult to get a suspected case of BPD into therapy----remember they refuse to accept personal responsibility.
BPD is the primary profile used by domestic violence advocates, etc.
Let us not minimize this disorder! Customarily, in the past men were labelled as Anti-social while women borderline....times they are a changing.....
Thank you for the thread! I'm glad to have stumbled upon it!