It's a combination of a few things, like Jeffrey Epstein convincing moot to create /pol/ on 4chan which led to 4chan switching from a mostly leftist platform to a mostly fascist platform over time, Gamer Gate and the online right taking advantage of that whole fiasco to convince people that the anti-establishment position was anti-feminist and right wing even though it didn't start off that way, and the DNC rigging the 2016 presidential primary against Bernie Sanders as was shown in the Podesta email leaks which Hillary distracted from by just talking about the source of the leaks instead of the contents.

Well you see. It all started with people making edits of Senator Armstrong as Donald Trump.