Too insecure to back down...
In comments on Facebook, to some of my former AF KorLing friends:
Actually, DRPK and ROK have shot missiles and artillery past each other, have had naval skirmishes, the North Koreans even sank a South Korean vessel not too long ago. But putting artillery on a civilian target is a clear escalation. The fact that they actually exchanged artillery fire is another big deal- they usually end up doing stupid shit like this by accident, and then with no immediate response forthcoming, de-escalate. This may have been accidental, may not, but it got an immediate response, and they may "have no choice" from a commie propaganda point of view, but to continue the pressure and escalate.
They're too insecure to back down.