I don't have an official diagnosis of BPD, but I spend quite a bit of time lurking on relationships forums, and I see this all the time. Whenever people have negative experiences with ex-partners, they frequently diagnose them as having BPD without having any deep understanding of the disorder. This is especially bad on reddit, where BPD is essentially synonymous with "crazy ex-girlfriend syndrome." The behaviors that are described could fall under any number of things: NPD, C-PTSD, attachment disorders, etc. (Conflation of narcissistic and borderline behaviors is *especially* common, it seems.) I don't understand why it's so commonplace for partners to diagnose loved ones with BPD when self-diagnosis is so discouraged. 
Frankly, I think a significant number of these people have issues with black and white thinking and accepting culpability themselves, but see BPD as justification for blaming every relationship problem on the other person. Moreover, having been in a toxic relationship with a verbally abusive ex, I know that the "proof" that these people give of BPD—e.g. always playing the victim, being overly emotional—are accusations that manipulative and/or un-self aware people often use to gaslight their partners into questioning their own sanity. In the context of a romantic relationship between two consenting adults (in the vast majority of cases), painting your partner as satanic without describing any role you may have had in perpetuating the dynamic, or describing the situation with any nuance whatsoever is a red flag in my opinion. 