Anecdotal, but my website lost a lot of search traffic after Google's core update in March, which seems to have affected SO as well looking at the first chart.

If I look at Google's guidelines, my articles follow all of them: in-depth, well-researched, demonstrating personal experience, better than other articles appearing in the search results. And yet, they were "penalized" by this update for who knows what reason.

I looked into it and some other websites benefited from the update, so who knows what changes they made and why.