A search of Usenet posts turns up some earlier uses of doxed or doxxed.
One use is figurative for “screwed, anally raped.” Urban Dictionary has this one too. The Usenet posts using this date to 2001. No clue what the origin is. It appears to be especially popular in the online gaming world, and might—I’m just guessing here—have something to do with casting spells in a fantasy game, as that’s the context a lot of the citations appear in.
Another is in reference to VHS tapes or DVDs, you get “doxed sets,” which I at first mistook to be a misspelling of “boxed,” except the phrase “boxed and doxed sets” is rather frequent.
I would suggest the first meaning above blended with the “documents” sense to give the specific meaning of private information being revealed.