There is close association between the American atheist and skeptic movements. Tensions between progressives and everyone else, especially atheist economic Libertarians, have exploded over the last three years, after a number of high profile incidents at conventions involving thoughtless casual misogyny, harrassment, and even rape. An alarming, very ugly, anti-feminist backlash has emerged centred around a handful of well known figures on either side of the rift and two websites. The only person you will probably know is the blithely privileged and ever more reactionary Richard Dawkins.
As for the web, Freethoughtblogs.com is an illuminating site. Slymepit.com not so much. Here you will find links, human and hyper-, to Breitbart, Donald Trump’s favourite news source. I’ll leave it to you to work out where SJW gained popularity.
There is further overlap between anti-feminist gamers and anti-feminist atheists. Gamergate, which was a non-scandal aimed at silencing and shaming females in the gaming world, further popularised the use of “SJW”.
A little glimpse into the online festering of American middle class prejudice that has exploded into public consciousness via Trump.