All because of that wretched unnecessary referendum.
A number of surveys have shown just how many people think violence and the break up of the UK is a price worth paying for Brexit (or no Brexit). We've already got increased tensions in both NI and Scotland. Brexit has all the far right racist groups on their side, so quick to violence, Remain has the young on its side, so again quick to violence. Politicians and the media seem to be intent on only raising the temperature further.
This isn't something that keeps me awake at night or anything but I think people are naive if they think it could never happen here. I suppose one way to avoid it would be if Brexit did happen and the sunlit uplands and all the Brexit promises actually happened. I don't know anyone (Leave or Remain) who believes everything will suddenly get a lot better anymore though. Even if it did I still can't see any solution for the border in Ireland.