Interesting - good point. Potential theories to explain all or part:
(1) there’s more new accounts than old, typically
(2) the lifespan of an account arguing politics on the internet is short - it’s too futile and frustrating
(3) good-faith sock puppet accounts (don’t want a non-anonymous record of political beliefs)
(4) bad faith sock puppet accounts (trolls, meanie-heads)
(5) confirmation bias - once you see it, you notice all the new accounts, even if the effect is smaller than you perceive
(6) new accounts are likely to be less HN-technical folks, as the site grows past its original audience, and they’re more likely to comment on non-tech posts in general
(7) the Russians?