When feminism and social justice get brought up on reddit in a negative light, it's almost universally (and accurately) talking about third-wave feminism.
Unlike first wave (which focused on legal right and suffrage, ~1900s-1930s) and second wave (which focused on job rights and gender equality, ~1940s-1990s), third wave feminism takes a much more aggressive approach to, well,
everything.
Whereas in the past feminism could be said to be for the promotion of womens' rights through the proliferation of
equal
rights, 3WF (which, unfortunately, has all but entirely co-opted the term "feminist" nowadays) eschews the concept of "earn equal rights" and focuses more on "reduce mens' rights." The role of the patriarchy and a men-oriented society is seen as a bar that needs to be lowered rather than overcome - rather than adapt and meet the norms of modern society, feminism seeks to force modern society to adapt to their
norms.
But how does this tie into SJWs? Well, it's almost synonymous, though "SJW" generally applies to a broader picture that includes women, LGBTs, and (for lack of a better term?) the mentally deviant (other-kin, transethnic, etc). SJWs and modern feminists, rather than striving to achieve equal rights for the groups they represent by proliferating them into society healthily, seek to do so by
forcing
others to repress any criticism or disagreement.
The application of this can be seen pretty easily, especially on the net. Take
/r/tumblrinaction, for example - while those are usually the very extreme manifestation of the SJW mindset, it's still accurate to a large degree. You can't question a person's self-defined identity, regardless of how nonsensical it is ("No, you are
not
a goddamn half-wolf half-elf spirit trapped in a human body"). You can't use certain words, because they're "triggering." You can't imply that men are anything other than suppressive, corrupt, sex-crazed pigs, because who
else
would be the source of our victimization complex? If you agree with them, good, if you don't, you're obviously a patriarchy-propagating misogynist (it gets even more hilarious if you're a woman who disagrees).
On reddit, this manifests as very harsh controls on a lot of subreddits - on
/r/games
and
/r/gaming, good luck if you try to bring up Gamergate, since even though it's about media corruption it's labelled as "misogynistic" and discussion of it is banned. On
/r/offmychest, "bitch" is a banned word. Comments, posts, off-subreddit discussions - more and more subs show evidence of mods following a Zeroth Rule of "We reserve the right to remove whatever content doesn't mesh with our political ideology."
Feminism (and its logical extension, "Don't treat people like a dick because they're different") is,
itself,
a good thing. I don't think you'll find anyone who disagrees that women/all people should have the basic right to equal opportunity and freedom from hateful discrimination.
However, third-wave feminism and the modern SJW movement take things too far - rather than opt for a gradual, healthy proliferation of feminist ideas by setting a social example and through due process, they take the goddamn nuclear option. By analogy, a healthy feminist movement would look something like Gandhi's liberation protests; the modern feminist movement looks something more like ISIS.