(FWIW: While I do generally enjoy DF, my interest here is primarily in understanding HN. I read HN probably 5-10 times a day, whereas I read DF perhaps 5-10 times per month. The near-absence of DF on HN doesn't affect me at all.)

    He did start to write a lot about US politics, 
    which for me is enough to stop reading his blog.

That makes complete sense to me. It would take only a very few "major turn-off" articles to make me remove a blog from my feed and/or stop visiting it directly. Even a 1% incidence of such posts could cause that blog to lose 100% of my traffic.

However, that doesn't adequately explain DF articles' swift removal from HN's front page.

On HN's page front page I'd expect article links to sink or swim based entirely on their own individual merit.