Have a source for that claim? I watch a lot of YouTube with one account and it only recommends things similiar to what I already watch. There's no sudden fake news switch to Fox news or CNN or... anything, really.
https://www.nytimes.com/2018/03/10/opinion/sunday/youtube-po...
It links to this WSJ investigation:
How YouTube Drives People to the Internet’s Darkest Corners https://www.wsj.com/articles/how-youtube-drives-viewers-to-t...
I suspect people who watch a lot of YouTube aren't as badly affected as there is a large dataset to recommend from.
The logic is simple: the more reactions a piece of content generates, the more it will be recommended. "Fake news", at least in the form of incendiary propaganda, is designed to invoke strong emotions. In the case of YouTube (probably applies to FB too), the compound effect is pure amplification.
In a way one could say that YouTube and propaganda thrive from a perverse symbiotic relationship.
0: https://www.theguardian.com/technology/2018/feb/02/how-youtu...
ß: You can easily find other news items on the same topic, but they tend to have either have a paywall or GDPR modal.