If you're looking for a specific set of causes re, the growth of RW populism, don't neglect the influence of malign state actors.
Aleksandr Dvornikov, commander of Russian operations in Syria 2015-16 said "The information war was on both sides. Its results were directly reflected in world public opinion. I will say this: without carrying out information operations, we would not have had success in Aleppo, Deir al-Zour and Ghouta." - by information operations, he means active disinformation - lying, smearing, fabrication. I think that part of the importance of the conflict there is that it's been a testing ground for these strategies in their newest information age iteration. And it worked - the propoganda war has kinda been won. Who has a clear view on Syria? Who really calls out the Russian forces for the war criminals that they are - compare it to how Western forces are viewed after any major conflict - the difference is remarkable.
And similar strategies are employed against The West - continually, in a myriad different forms. To take an innocuous seeming example (that took place in Russia), I'm assuming you all say the fake video of the women spraying bleach on the Moscow tube? What purpose does this serve other than to discredit feminism and by extension the whole body of liberal/leftist/progressive thought. And that's just one very small example. For operations directly affecting the West, one might look at the fake accounts associated with Black Lives Matters and similar causes. If you look at the budgets for this stuff - the kinda information now emerging via sources like Bellingcat - they're absolutely huge.
I'm not saying we don't have huge inequality, massive social fissures, already in place. And I'm certainly not saying our politicans have been working hard to fix these divisions - 'cos they haven't. But I do think they are exacerbated and made much worse by outside forces. It's probably too early to make a totally clear call but I suspect evidence is going to emerge showing both Brexit and Trump's elections as the most striking examples of this.