The algorithms I believe. Both active and hot are currently a bit broken, as are a few other things. Their biggest problem is they have trouble deprioritizing older posts, which leads to them getting more activity, keeping them high on the prioritization list. I've found sorting by new comments, new, and top (today) are really good ways to explore the site, and I've heard that the Lemmy devs are working on a fix for the broken algorithms. You can also hide read posts in your settings, though that has some drawbacks.