It’s mainly a mix of economic frusteration and a divide and conquor strategy by the rich in order to have the masses blame immigrants and people of other races/religions/genders for all of their problems instead of blaming the elite class and capitalism.

There is another angle, though, and that’s sexual frusteration. When a teenage male has issues getting laid and they look online for some kind of support, they find people like Andrew Tate and Jordan Peterson who tells them that men who “get laid” are “alphas” or whatever and that women should be treated as subhuman or as slaves. These incels have likely never actually attempted to make friends with any women outside of a sexual context, because if they did, they would learn that women are the same as them, even when it comes to their struggles (even pertaining to dating/sexual frustrations).

I think the latter reason sparked the fire that made the first truely possible on a scale that we haven’t seen in our lifetimes.