I suspect a significant factor in the US is the influence of the internet. This can be used to amplify the voice of small, anonymous groups and to create echo chambers that become self-reinforcing. Prior to the internet, when a typical person espoused a crackpot conspiracy theory, their buddies could be on hand to keep them well-grounded. That is often no longer the case, since people will head to the internet to anonymously spout rubbish. It doesn't help that foreign adversaries can help promote this behavior, to an unknown extent. We're more likely to be targeted by these malicious agents due to our foreign relations.
I'm not sure yet what we can do about this.
Another factor has been the isolation brought on by lockdowns, social distancing, and mask wearing. While necessary because of the lack of a vaccine, these measures are harmful to the human psyche. We need social contact. If a person is close to the edge psychologically (including the 1% who display psychopathic personality traits ), this can completely knock them over into self-delusion and potential violence. Some of this may have been reflected in the rabid, illogical anti-masking posts I frequently see. Maybe. I'm hoping this effect will gradually fade as we open back up again.
I completely understand the desire to rid ourselves of these quarantine measures, but I also agreed with their temporary necessity to prevent mass deaths.