It's affected by algorithm changes. For example, the software now downweights any story that's 18 hours or older, with the intention of flushing the front page so that the stories with largest inertia don't just sit there.

I doubt it's that users are less engaged. It appears that HN had its most active Sunday ever today in terms of unique visitors. I've been meaning to look at some graphs of number of comments over time because it feels like those have gone up lately, too. If anybody else wants to make them, the data's public...