I don't know, but HN has clearly begun to skew more and more to the "left" (in the modern, American sense of the term) over the past few years. I'd say it's been really marked over about the past 2 years. Why that is, is hard to say.
While it doesn't explain why, the observation of Robert Conquest's 2nd Law of Politics does seem to be correct: "Any organization not explicitly right-wing sooner or later becomes left-wing."