Why? Well, here in the 21st century, we are developing new slurs because the old ones were not good enough? For those not in the know, SJW refers to Social Justice Warriors, and has largely been used by the misogynists of the gamergate community to refer derisively to those who advocate for gender equality and other "libtard" stances.When I see folks use SJW in their tweets, I assume tweeter is a misogynist. If you don't want to be associated with gamergate, then don't.— Steve Saideman (@smsaideman) September 18, 2015
Which brings me to the second 21st century slur trend: adding -tard to something to refer to an individual or group of a particular ideology or whatever: libtard, contard, whatever. It, of course, is an attempt to conflate the individual or group one disliked with retard suggesting that these folks, whoever they are, are mentally handicapped in some way. This is, of course, offensive to both those who have mental or emotional problems and to the targeted group.
The use of either SJW or -tard demonstrates a willingness to use shorthand of a particular kind to signal that one is part of a larger community--those against SJW's, those against liberals or conservatives or whatever--and that one hates the targeted group. Yes, hate. Because these terms are intended to be derisive and chock full of contempt.
And since I am no Gandhi, I tend to have contempt for such folks. So, if I see someone using SJW, I will assume they are misogynists as they are identifying themselves as part of the larger group of people who have contempt for women and for those who advocate for equal rights. Because, yeah, I think social justice is mostly a good thing and the opposite would seem to be social injustice, right? And if you use -tard, then I am assuming that you are ignorant and hateful. As it has become socially unacceptable to use retard, I am not sure why using part of it is seen to be ok.
If this offends you, then, well, sucks to be you. And I would apologize if I offended you but that would be that super-insincere non-apology apology.
Update: I was reminded by a friend that I am guilty of this. I concur and will try to be more self-aware and do better.