I've only had that happen once and I decided it was because he was a psycho. Of course there are all different degrees of anger but usually my experience has been one where the guy is only visibly disappointed but not angry.

I see a lot of men here implying this is a level playing field. It's not. There is no female equivalent of the Incel movement. When a man expresses hostility toward us for rejecting us, we have reason to feel threatened and to fear that he may take that hostility further. In many cases, that open hostility the man expresses can be just the warm-up, and we have no way of telling if that's the case. Men may joke about it, but they seldom have to wonder if the woman they just brushed off will be waiting for them in the parking lot to force themselves on him, injure him, or murder him. These are real concerns for women, and there are stats to prove it. Of course "not all men" are monsters. But the monsters are indistinguishable from the rest of you. Men fear getting screamed at or having their masculinity questioned, if they insult a woman. Women fear being stalked, maimed, and murdered for insulting a man.

I’m sure it happens. But, I’ve not seen it.

In simplest terms, men (some) are insecure and take rejection as an attack on them. So, they react (immaturely) by attacking in return.

Not one of our finest traits.

I have no idea. Never done that in my life.

I do not get that. As a human species, we are all attracted to different people. Sometimes it is a match, others it is not and that is ok. No need to be rude about it. I think a thank you for your candid reply is the best response.

There are certainly many reasons that rejection may manifest in anger. Anger is an easier emotion to deal with as it can be directed outward whereas rejection causes internal questioning. "It isn't me it is her".
As for the expression of that rejection in profane and/or derogatory comments, which we see often in internet postings, they may be the result of the person not having the ability to express themselves in any other way and the lack of consequences for such expressions. These people seem to have a need to have the last word.
Our society has become more accepting of crass behavior, the rise of women demanding a voice and equal treatment in a male dominated and privileged society and ego issues have been discussed. And all may a part in why name calling is acceptable to those who choose to do so.
Having said that - the problem will not go away. People are what they are and each of us has to know that such behavior is not our problem - but theirs.

I see it as a sense of entitlement, something that is becoming more common with the advent of social media and the anonymity that it provides, so people feel free to express their baser instincts. There's a lot of discussion lately about toxic masculinity, which I suspect comes from a lack of emotional intelligence.

There is an easy answer! Some "men" are actually very immature!

Most men just wanna fuck plain and simple

Their brains are immature

Their pride is hurt. when you reject a guy you crush his ego it's basically a defense mechanism

The best thing you can do is avoid them