It had not occurred to me before, but the pejorative aspect of 'SJW' might be somewhat amplified by the (most likely unintentional) reference to the 'Warrior of the Light' stuff from Paulo Coelho (which I've only ever read excerpts of but, having tried to go through The Alchemist once, they were enough).
More to the point, my guess is that its effectiveness mainly stems from the use of 'warrior' as the defining feature of an individual (almost elevated to a profession). I.e. implying the person is someone who fights because... it's what they do.
Simultaneously, to some it may evoke the image of the romantic (here refering to the chivalric ideals) knight and the associated naivety world view and crudeness of method. Possibly further reinforced by the ineffectiveness of action at the knight-level to further causes of social justice.
All in all it seems a subtle and well-thought out invention. With hindsight, I can see why it would catch on and I'm not sure I'd want to reclaim it. Though if it becomes dominant, then it /has/ to be reclaimed one way or the other. Hopefully we're not there yet and some better term can be pushed in its place :)