One of the key things with BPD is a persistant pattern of a given trait. For example, let's take fear of abandonment. Being afraid of losing a small number of people over one's lifetime, especially in certain understandable situations, is normal. However....

With BPD, though this fear might not include everyone, it is more extensive in scope. And it is an active fear, in the constantly scanning the horizon for any clues it might happen. These clues may be real or imagined. Perceived slights can be blown way out of proportion. These fears are usually accompanied by certain reactive behavior, such as provoking trouble over them as a means to "test" the person's loyalty*. Thing is, it is the constant "testing" behavior that would drive a person away. (Being constantly tested to "prove" oneself against unfounded threats and slights is exhausting, for starters.)

(It also might take the form of dumping the object of their FOA out of the blue. So much fearing the abandonment, they employ a pre-emptive strike. Very often followed by desperate attempts to reunite. Then break up. Lather, rinse and repeat till the other person can't take it anymore.)

And voilá, they leave, thus providing "proof". Except that it isn't. Nonetheless, this creates a scenario that happens again and again, establishing the pervasive pattern. But the whole premise is wrong. It's NOT the perceived threat that caused it, it is the behavior it produces that does. Is it any surprise this cycle makes for unstable and tempestuous relationships?

(*I often think of this as "throwing wrenches". Everything can be going along fine, and boom! A crisis out of nowhere.)