It's the ongoing increase in partisanship. It makes it easier to form ideological bubbles even where they didn't exist before, because the ratio is so skewed to begin with, it's entirely possible to end up in a group of people selected by some unrelated criteria (e.g. a work team), and find out that there's a significant overlap ideologically. So issues that were not okay because of the potential to offend someone etc, gradually become acceptable, because (at one particular point) there was no-one in particular to be offended. Of course, once established as part of the culture, this becomes self-reinforcing...