Clusters of very old posts in the hot feed

What’s up with the super old posts in the hot feed? They always show up near the top in strange clusters, too. I’ll be scrolling through, seeing posts from 5 min to an hour old, then suddenly 10 posts in a row from 2 years ago, then back to seeing all posts from 5 minutes ago. I know I can sort by top day or whatever, but it...