It’s really simple. People don’t want to mix.
Diversity + Proximity = Violence.
That is most always true, I think.
I'd like to add another ingredient, and (as in chemistry, more so than math) it may just be a catalyst: Diversity + Proximity + (Ideology) = violence.
You would have to define diversity in a way that affirmed intrinsic incompatibility between groups for that equation to hold.
So which groups are incompatible together?
What are the reasons for the incompatibility?