I think it's rooted in attention, and stems from having been hurt by something or someone. That person then focuses on the perceived injustice and can either score quick internet points with others, or else vent about whatever seems to be a "symptom of the problem" as they perceive it. These people often don't realize that everyone has been hurt by some other person or group in the past. And that the healthy thing to do is let go, rather than try to control and change some broader thing that is bothering them (because they can't). But it's a trend, for sure.
It seems like a Gen Z thing. Older generations know the world is so far from perfect anyway that it's like you know what, just try to deal because there are bigger fish to fry. We're also used to nobody giving us any time for our problems. We never really got to have any group traction with regards to a minor gripe of ours. We just had to put up with it.
It's also a way to stand out. If we can't be rich, or beautiful, or anything else desirable then we can at least try to one-up someone else in terms of morality. But despite all that, these people are often some of the least pleasant to be around.
It only really became a thing with people high-fiving each other over this stuff on social media. Without at least a little validation from other people saying "you're right, that IS wrong" this would never have taken off.