SJWs are the current pathology du jour, and Vox Day has written an excellent book on how to fight them.
Unfortunately, it is only for how to identify and fight them, not so much as why there are SJWs.
In the early days of Gamergate, there was an interesting dig/testimonial done by someone who had hung around SomethingAwful for a really long time, and witnessed its transformation from a site and forums that poked fun at anything, to a safespace hugbox that has an essentially protected class not allowed to be made a mockery of.
The term SJWs comes more immediately from Tumblr, where they self-identified as warriors for social justice and Jim, known then as Internet Aristocrat, now as Mister Metokur, devoted a numerous amount of videos making fun of them and almost singlehandedly spread the term of SJW and associated it with mockery. Then Gamergate happened, and the rest of the world found out about how pointless SJWs are.
Tumblr was always a stronghold for special snowflakes however, which is why it's more interesting to look at the SomethingAwful case. In many cases, the very same people who were for mocking everything were now ardent Social Justice Warriors. It wasn't even a clear-cut instance of entryism, but rather conversion.
To cut right to the point, I've determined that SJWs exhibit their symptoms because they are constantly denying reality. They always lie to themselves and others because the truth completely upends their religion and worldview. They always double down because it is their religion and worldview. It is also the only concept with which almost all of them are familiar with: they are basted in Social Juices since preschool, taught that equality is uber alles, while constantly avoiding the critical thinking that would show that equality is impossible. They're taught that women and men are essentially the same, just with different fiddly bits, taught that war is never ever ever ever the answer, and taught that competition is bad. And by extension, SJWs always project because A) they very often don't know anything else (whether modes of attack, other forms of ideology, or just that other people actually ARE different), and B) are reacting emotionally because SJWism can't be maintained logically.
They're so stressed and triggered and unhappy because they're constantly fighting reality. If something happens that contradict their view, their entire world is rocked.
Now, back to SomethingAwful... there was still a strong undercurrent of feminist modes of thinking from public school in the forums and site. They'd "wrongly" make fun of things that were taboo, like 9/11 a couple of days after it happened, but they still had that sense that it was 'wrong' and making jokes about women being in the kitchen was 'wrong'. It's really why they found it hilarious.
After raiding Tumblr a couple of times because they were a bunch of crybabies, said SJW concepts infested the feminist mindset that SomethingAwful had. There was nothing incompatible with it, in fact it was an outgrowth of sorts. Everything SJW was what feminists had taught, just taken to a slightly higher degree and being applied to everyone. It was radicalized, and it appealed to the guilty conscious of SomethingAwful (because yeah they made fun of taboo stuff, and they knew it was wrong). In short,
SJWism is radical feminism.