Watch Da Birdie
I buy cakes for myself on my birthday it's not weird lots of people do it I bet
Concerning the rise of "Social Justice Warrior" as a concept: I've always thought it had to do with the idea of "white knights" that was prevalent for a while in certain circles, though you don't see it used as often anymore.
The sort of people who would use a term like white knight generally seemed to think that anyone they were engaging with online was also a man -- male being the default, apparently -- and that any man who would be concerned with women's issues must be doing it in the hopes of getting laid. (This is just more of that general idea that men and women can never be friends because women are only objects of desire, who either acquiesce or "friend zone" the men who are the ones with agency.) A woman couldn't possibly be arguing on behalf of herself, it must be a goody two-shoes white knight whose only goal is to obtain her.
Once they began to realize that some of the people who disagreed with them were women themselves, they obviously couldn't be called white knights who want to obtain other women as a prize -- lesbians after all only exist for the male gaze -- but the idea of sneering at a warrior who makes their career out of a naive pursuit of some childish idea of justice (mixed with a suggestion of self-righteousness) still appealed to them, and so the idea of women as Social Justice Warriors took hold as a particularly snide way of harking back to the old "professional victims" refrain. The term has since grown to encompass men as well, and basically anyone who disagrees with you on issues you deem to be "politicized."
Just a theory, though.
I mentioned this yesterday, but the original definition of Social Justice Warrior was ironic, and used to refer to Internet activists who complained about social justice issues, but never actually did anything except scream and attack others. Furthermore, it usually involved completely inane issues that detracted from real-issues, such as corrupting trigger-warnings into a contest to have as many as possible for stuff like "shipping" and "bad food", sexual identities that were nonsensical and diminished the real issue of gay rights and trans rights, such as being "transethnic", "transhandicap", and one sexual identity where you only had sex with people you found attractive (so, basically, a normal individual), and generally, just silly stuff. There was a clear distinction between people who actually supported issues like feminism, gay rights, etc., and Social Justice Warriors, who didn't really care about any of that and just liked the drama. However, soon people just started calling everyone they disagreed with SJWs, and soon the term evolved into a legitimate term to describe actual Social Justice activists.
As you can see, it's the GameGaters who are the REAL "Social Justice Warriors", going by the original definition. All they do is attack others for completely inane reasons and ignore the real issues---none of them are actually concerned about ethics in journalism, they just like to hate. And like the original SJWs, most of them are probably kids with too much free-time.