It's another one finger gesture from the electorate to their leaders, a real "fuck you, we'll do as we like" sort of gesture. Just like in the UK, there's a rebellion among ordinary folk
Based a lot on fear with a good dollop of ignorance thrown in. I know quite a few people who supported Trump. They aren't bigots, or sexists, or even hard core right wingers. But they feel disenfranchised as the world changes around them. America is a big country, and the 'progressiveness' of the urban centers does not reach out into the rural areas where most people's lives haven't changed for generations. Due to a certain degree of ignorance, they fear new ideas and things unknown to them, and they buy into conservative rhetoric. Throw in the baggage that came with Clinton (and she did have quite the luggage compartment), and then when that candidate calls them 'deplorable', they get their back up. And just as moderate, well meaning Republicans held their nose and voted for Clinton, many of these decent, hard working, backbone of the working class folks revolted against the political machine of Clinton, held their nose and voted for Trump.
It is sad to a degree that many of the decent people get swept up into the 'depolorable' basket along with the real bigots, sexists and hatemongers. But OTOH, if slop around in the mud with a pig, you might get mistaken for one.