Some free-wheeling folk converge on Tumblr, and start calling themselves warriors for social justice.
This spills beyond Tumbr.
Their behaviour, though effective, is so abhorrent that SJW immediately becomes a term of abuse.
SJWs try to backpedal from the name and get on the euphemism treadmill, but their antics keep them in the spotlight, so nobody forgets. They can't help themselves. They constantly deny that they use the tactics they in fact use, but can't even coordinate enough to produce a lull to let the backlash subside a bit.
Vox Day instigates a list naming SJWs as SJWs, where inclusion requires they perform the kind of actions they openly call for.
Social Justice Warriors initiate plan: losing their minds.
Sophists everywhere are terrified of anyone calling a spade a spade. Their tactics are effective in the short term.