The Cooperative Congressional Election Study (CCES), an election survey of about 50,000 people, found that 12% of Sanders voters voted for Trump in 2016.^[https://www.washingtonpost.com/news/monkey-cage/wp/2017/08/24/did-enough-bernie-sanders-supporters-vote-for-trump-to-cost-clinton-the-election/] In the states of Michigan, Pennsylvania, and Wisconsin, the number of Sanders–Trump voters was more than two times Trump's margin of victory in those states.^[https://www.nbcnews.com/politics/2020-election/sanders-voters-helped-trump-win-white-house-could-they-do-n1145306]

https://en.wikipedia.org/wiki/Sanders%E2%80%93Trump_voters

Why wouldn't it happen? There are millions of people in the US, and a lot of them aren't very smart.