Machine learning probably had something to do with it. It notices videos with thumbnails of people with their mouths and eyes wide open doing some kind of shocked expression with red arrows pointing to something perform better than average. Then it starts recommending videos with thumbnails like this preferentially. YouTubers take notice and start producing thumbnails like this for every video. Go to the front page r…
Notably though, if I log in and visit my home page I have only one video of this nature on the front page in the first ~50, so it does appear possible to teach the algorithm you will not engage with such content even without conscious effort.