Is it common that people in a midlife crisis enter relationships with BPDs?
I think it's probably more that being emotionally vulnerable makes one susceptible to engaging in
an unhealthy relationship with a pwBPD(OW), who steps in to fill the void and to bandage over the hurt.
Certain aspects or common denominators are present in males who attach to Borderlines.
Generally, these are People Pleaser types, who have rescuing or fixing compulsions, self-esteem difficulties from childhood, intimacy issues, engulfment concerns, poor self-image, dysthymia (chronic/long-standing mild to moderate depression), etc. Foundational problems of this kind leave men vulnerable to being seduced and manipulated by these women. You may be extremely accomplished and successful--but the Borderline will methodically learn what's underneath those props, and use your most intimate secrets and self-doubts against you.
Every man's ego takes a jab when a woman leaves--but a Borderline leaves you feeling guilty, ashamed, castrated, unlovable, emasculated, worthless, etc. You start believing that if she returns, you'll be able to get rid of these horrible sensations, and feel okay again.
You might think of her like a drug you can’t live without, because you've felt alive and buoyant when she was attentive, available and loving, and tortured and empty when she was indifferent, detached or cruel.
There's always a childhood template that sets up our attraction to someone personality disordered. Perhaps your Borderline has traits similar to mother or father, and you're familiar with the relationship dynamics you've struggled with in this attachment (which keeps it exciting, despite all the pain).
You'll keep wanting to blame yourself for this relationship faltering, but this is directly tied to experiences in childhood, which left you with self-esteem wounds. A young child can't make sense of why he isn't getting enough love, affection or support from a parent, and he doesn't even know how to ask for it! He'll try to find reasons for this lack of attention in his head--but the only rationale he can come up with is, it must be his fault; "I must not be good enough, smart enough, cute enough, lovable," etc. You've carried these self-worth injuries into your adult relationships, and now they're alive again.
NOTE:
BPD usually develops during adolescence or early adulthood.
In addition to sexual trauma being a factor in the development of the disorder, parental neglect and unstable family relationships has been shown to contribute to an individual’s risk for developing this disorder. Other studies suggest BPD may also have a genetic component; it is thought that individuals may inherit his or her temperament along with specific personality traits, particularly impulsiveness and aggression.