In the case of YouTube at least, to me this seems like it's probably overcorrection being done to try to manage the particular brand of extremely over-the-top negativity that's commonly seen in the comments section. Comments like this often require moderation for being abusive or otherwise breaking ToS but can't be handled right away, so they bubble the positive comments up to so at least the questionable comments are buried a bit and don't get as strong of a bandwagon effect.

If this is what's actually happening, it's a case of particularly nasty commenters ruining it for everybody.