Well, apparently they changed the way related videos work. Now they work it out from what you've watched earlier instead of actually related to the one you're watching.
I'm for example getting this one on the Giddy Up chorus version www.youtube.com/watch?v=w4uuV5V73sg
I can't really remember the last time I watched borderline hentai on YT, but good to know that YT thinks I want to.