Most people are pretty rational about most issues when you discuss them individually, but you end up with a few ''foundational''-- and unquestionable --ideas that people have to fall exactly on one side or the other. I won't name any specifically but I think everyone knows some of these immediately. So you have individually rational persons who have to congregate on either one side or the other, and these issues end up being the deciding factor of which group you must join, dividing many people who otherwise agree on a lot of stuff, perhaps without even realizing it..
It's pretty similar to religious fracturing to me now that I think about it. Groups who agree on everything except one or two ideas and that makes all the difference.
Very rare are the persons who fall heavily to one side of everything.