I was thinking something similar: SJW started as a label for a very fringe group (see claims of white / cis / male being worse and not equals with peoples of other race and identity). However, it has since become a pejorative to refer to liberal activists of all sorts. Attended a march for worker rights? SJW. #blacklivesmatter? SJW. It's a little like "hipster" which itself started as a term for white people emulating black jazz musicians and has since been extended to include knitting.SJW = hipster in the fashion that it's a term used an insult to toss at "those not like me", in this case typically due to views on feminism and trans issues, amongst other similar topics. This was a term typically applied to those within a cadre of people who take a militant stance about social justice issues, typically far and away on the fringe, politically.
The ability for the internet to make anything into a pejorative is disturbing. It's mostly kids doing this but responding to stupidity with more stupidity is what the internet does best. See Anita Sarkeesian. She wants a more diverse representation of women in vidja games but clearly didn't play through Hitman. Internet court says the verdict is rape and murder threats
To be fair, it happens outside the world of the internets. O'reilly uses "liberal" as a pejorative in the same way that others might use the word "asshole". But anonymous threats, swats, and doxing + bullying are sadly more common among pissed-off internet kiddies.