I have experienced something very similar with my mother. She is college educated, pretty smart, and I really didn't see it starting.
She is 68 and that seems to be a common thing. I really think it starts through Social Media and the inability to separate actual news from truly fake news and Conspiracy. Also, women seem to be much more susceptible to groupthink. They seem more likely to parrot talking points that their girlfriends spout and less likely to disagree with the group. I use the #savethecildren movement that we saw a few months ago as an example. All of a sudden, many smart women were suddenly convinced that there was a suddenly, huge problem with child sex trafficking that was occurring. They couldn't explain why they felt this way, they were just retweeting and reposting away and getting more and more upset that "nobody cares about the children".
What's crazy, is that it's almost completely gone now. Hopefully they will come back to the real world soon.
My mother is finally showing some signs of normalcy now post election. She has started trying to post nothing but positive things since she realized that she was living in a very negative place. She isn't ready to admit that Biden won, but I think it's coming