Expression and suppression of ideas have been happening ever since the onset of language in human, as far as I can tell. Any explanation that is time dependent, i.e this is happening because of so and so happening currently seems a bit inadequate to me. Any type of "look how crazy the other side is" rhetoric also feels like cliche to me because who am I do decide what's insane and what not?
As for the core of the problem itself, I have no idea what the core of the problem is. Vaguely, it seems that people take a side of issue that's phrased to have two side and they duke it out with the other side. It seem to be part of underlying mechanism of how humans behave because I see similar ideas played out in sports and such.
Maybe the problem isn't any one specific idea but rather how ideas get parsed by our brain. Or maybe I am the one complicating things and it's a simpler idea. But this whole business of right or wrong ideas, sides, values etc seems exhaustive. I'll just wait for someone else to figure it out for me.