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.
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.