Here is the Urban Dictionary definition that does a better job of describing what people don't like about Social Justice Warrior.
[P]"A pejorative term for an individual who repeatedly and vehemently engages in arguments on social justice on the Internet, often in a shallow or not well-thought-out way, for the purpose of raising their own personal reputation. A social justice warrior, or SJW, does not necessarily strongly believe all that they say, or even care about the groups they are fighting on behalf of. They typically repeat points from whoever is the most popular blogger or commenter of the moment, hoping that they will "get SJ points" and become popular in return."[/P]
I underlined the key aspect. It is people who rush to be the first to point out (or share) a rumored injustice before giving any reasoned thought to the situation or having enough facts to actually know that an injustice has occurred or what the nature of the injustice is.
People that sincerely care about injustice want to reduce them. This goal is objectively harmed by false positives of "injustice" and inaccurate assessments of what the nature and real cause of it is. Since avoiding false positives and inaccurate understanding of injustice is only possible via careful evidence gathering and reasoning about and event, those who act (which includes expressing outrage) before they plausibly had the time and available evidence to make these determinations must not care sincerely about reducing real injustice. They are SWJs. The label works as irony. People that care about injustice generally don't try so irresponsibly hard to show you how much they care about injustice. They essentially are labeling themselves. That is their ulterior motive. This irony of the term is so transparent that Nexus is quite right that anyone implying that anyone using the term pejoratively must be against actual social justice is being dishonest.
Basically, an SJW is someone that abuses the cause of fighting for justice as a game of one-upmanship. Much like the person that seeks to show their superiority by being the first to adopt a commercial trend, the SJW seeks to be first to adopt a new outrage. Also, you get double the points if you not only protest a presumed instance of something that would be a legit "injustice" if it had occurred, but also promote the idea that everyday interactions that are not injustices should be classified as injustices (as is the case with much of what falls under cultural appropriations and micro-aggressions).
Obviously, conservatives are going to latch onto the term and abuse it discount any and all claims of injustice. But, as with "politically-correct", that doesn't mean the term does not refer to a real and problematic tendency among many on the left who do more to distract from actual injustice and to fuel the conservative base than to promote justice or fight bigotry.
