A variant is that Democrats drove "working class" voters away by showing contempt for them. This is more plausible, but raises the question of whether Democrats showed that much more contempt in 2016 than in 2012, 2008, 2004, etc. That seems like a hard case to make--at any rate, I haven't heard anyone try to make it.
So why are these explanations so popular? My hypothesis is that it's because American society has become a lot more socially egalitarian over the last 60 years or so. Educated people don't want to be thought of as snobs or elitists, and less educated people are less likely to think they should "improve themselves" by emulating the middle class. At one time, you could say that Democrats thought of themselves as the party of the common people, and Republicans thought of themselves as the party of successful people. Now both parties think of themselves as the party of the common people, plus the fraction of the elites who care about or understand the common people. The result is that people are attracted to an explanation that is more flattering to the "working class." When thinking about this, it occurred to me that I've seen many books and articles on how the Republicans can win over working-class voters, but nothing on how they can win back the kind of educated people who used to vote Republican. That is, gaining working-class voters is thought of as a more worthy goal than gaining middle class voters.
There are two possible objections to my account. First, it's easy to point to examples of condescension and contempt today. My reply is that there was probably always a lot of this in everyday political discussion, and that social media has just made it more visible for those who are paying attention. A second is the recollections of people like Charles Murray (Coming Apart) and Robert Putnam (Our Kids) about how there used to be less social distance between classes. I think that may be because they both grew up in small towns in the Midwest. If you read something like E. Digby Baltzell's The Protestant Establishment, you get a very different picture of status differences in America.