I have a very much subjective theory that the richer you are than you are smart, the more conservative you are. And likewise, the smarter you are than you are rich, the more liberal you are. This assumes that someone who is ridiculously wealthy but fairly unintelligent would be strongly conservative (true in practice, mostly), and that someone with many degrees and a very high IQ who is not financially successful is strongly liberal (also mostly true in practice). Obviously exceptions on both sides of the fence, but this has held true better than other predictors for me. So the fact that there are tons of world-aware, well educated people with minimal financial growth would be a recipe for liberalism.
This theory has problems that go way beyond subjectivity.
I would put my money down on 3) Insularity as the biggest driver of polarization over the past 20 years.