Just the usual reason over there - intolerance of religious, sectarian beliefs etc, plus it is what the neanderthals there refer to "the marching season" culminating in the 12th July "celebrations" (if you're Protestant), or most hated time of year (if you're Catholic), commemorating a battle more than 300 years ago, is the excuse for one side to be triumphalistic over the other. Although the "triumphalistic" element call it "celebrating our culture". On the other side of the coin are the Catholics who hate being part of Ulster Britishness and who want to instead have a united Ireland. They also go around at times banging drums and playing instruments, too.
All pathetic, really, just like the (in)famous Drumcree, near Portadown, carry on, where there are barricades in place to stop the Orangemen marching down a so called Catholic area to make a racket banging their drums and playing their flutes. That particular march has been banned since 1998 because of the riots it used to provoke.
It's basically total mistrust - loads of them live in their communities behind very high walls separating them from the hated neighbours, depending on which side you live.
So, nothing which won't still be going on in another 300 years. And this is the 21st century? Could have fooled me!