It's been observed that leftists, especially die hard feminists and SJW's, tend to act not only like rabbits (r/k theory), but more strongly than average exhibit traits associated with cluster-B disorders such as Borderline Personality disorder, narcissism, etc.
One trait of borderlines is, ironically, a lack of borders, more frequently, boundaries, between people. Dr Tara at shrink4men.com refers to this as plugging emotional hoses into other people, especially their kids, which is how you end up with a bunch of people-pleasing codependents.
So borderlines and leftists share a reduced sense of boundaries. Part of this is also enshrined on the left in the concept that the personal is political - that there is no inner self not subject to scrutiny and judgement by those outside.
Suddenly, it isn't surprising that they rarely accept borders between cultures either. Even with splitting, the tendency to suddenly decide someone is all good/all bad - it's not a matter of borders between people than "exists" and would prefer doesn't exist. It's a lot harder to tolerate someone that you do not view as actually existing outside of yourself, doing its/his/her own thing, separate from you, not reflecting on you.
Also, an aside. If you want to see a thoroughly chilling movie go watch Gone Girl, it's not as farfetched as you'd think given examples I've seen posted of meltdowns, including a mother-in-law shooting her daughters ex and getting caught on tape (they didn't know the guy was recording) preparing a story about how it was self defense.