Time to burn some karma.
SJW is not a term that is adopted by people who tend to do actual social good through campaigning and proactive behaviour. SJW (particularly on social networks) is a self-applied term that armchair experts use to make themselves appear to be "good" people, i.e., those who sign an ill-devised online petition once in a while or fervently use a hashtag to "raise awareness". Thankfully, for the most part, their reach is limited to a relatively small group of people who already agree with them and mirror their behaviour.
Well identified SJW behaviours:
Spewing vitriol onto the internet with little consideration for persuasive argument.
Adoption of any claims that reinforce their ideology no matter how poorly founded and conversely...
Rejection of any evidence, regardless of quality, that undermines their ideology.
Rejection of persons from their group who do not absolutely conform to the ideology.
Labelling persons - regardless of the quality of their arguments - who do not follow the ideology as problematic or racist/sexist/misogynist etc.
Adoption of any and all tactics regardless of how repugnant to further enforce their chosen ideology on others and destroy persons who do not conform.
Thus, SJW has become a pejorative because it does not describe people who do good, it is a self-applied label used by people who claim to do good while behaving in a destructive manner.