I imagine for a given topic you wind-up with:
[broad video] related to
[other broad but unrelated video] plus [extremist video].
And
[extremist video] related to
[other extremist video only]
Since youtube's recommendation system is entirely naive, once you choose [extremist video], that is the video that gives the system the more specific clue and thus [other extremist videos] will be what's recommended.It's a function of naive recommendations as such. If a system knows that X likes two videos, one that the entire population likes, one that only "metal heads" like. What can it recommend? Metal is the only sensible thing. And if the system then shunts many people to metal and they seem to like it, metal will count even more as a logical recommendation.