Easily turning on others who they disagree with or who disagree with them, who intimidate them, or who inadvertently scratch their pride is a common behavior seen in those with BPD, NPD, and other disorders that have hyper-defensiveness as a trait. Trauma, or prolonged trauma, can bring this on, PTSD sufferers may feel unsafe around those who don't seem to be an ally, even if the person is actually neutral or well-meaning. This is not their "fault", it's an effect that can heal when the person is surrounded with consistent allies over time who don't retraumatize them and invalidate them. People are either "on my side" or "not on my side", either "for me" or "against me". There is little or no middle ground, no room for negotiation or tolerance for other points of view. "Not agreeing with me means they are putting me down and attacking me."
A person who has a different perspective or who disagrees can express themselves politely, with the other person's well-being in mind. A person who gets off the topic and insults and attacks the other person is not disagreeing, they are insulting and attacking.
The confusion between the two (disagreeing vs. personal insult or attack) seems to be the crux of the heightened emotional reaction and hostility.
Disagreement is a civil, polite exchange regarding points of view about a topic that don't seem to match, there is nothing personal or hostile about it until one person makes it that way by focusing on the other person (you're dumb for thinking this, you're crazy, you're unstable, you aren't a good writer, you think I'm dumb because you don't agree with me, you don't care about me/you're stupid if you don't abandon your point of view for mine) instead of the topic. A polite disagreement also listens to the other person, and looks at the subject as much as they are able from the other person's point of view, and then discusses the differences in the points of view.
In many, even most, polite disagreements, after seeing the subject from the other person's viewpoint, both participants often change their original point of view and incorporate some points from the other that they didn't see before. This civil and interesting information exchange is how civilization and progress is made, and positive connections are formed and maintained.
Insulting is a bully tactic, used by the person who turns the discussion into a competition for domination instead of a pleasant exchange of information.
Some people are so used to using discussion and debate as a sparring challenge instead of information exchange that they don't realize that not everyone does that.