Though it is like reading leaves, there is good chance that this attack is directly related to the transition of power going on in NK right now. Kim Jong-un, under Daddy Kim's watchful eye, is likely deliberately using this attack in order to mobilize the military and consolidate his power.
The humanitarian food aid that had been sent to NK has ended except for whatever is coming from China, and that may now be suspended, even if just temporarily. While it was ostensibly to be used to feed a starving civilian population, it was in fact used to keep the NK military fed. They are now going to be hard pressed to do that. The civilian population in NK will likely die even faster now.
The current government in SK is more hard line than it has been, but you can't be really considered hard line unless you do more than chatter. Expect some additional retaliation.
Reading the international reaction, it is notable that Japan, also subject to North Korean provocations, is calling for retaliatory strikes. For their own security they need to make sure that NK is contained.
The real issue is the state of instability. As the various militaries respond - NK, SK, UN, US, Japan, China - you get a greater chance for escalation though it is yet unlikely that this signals the start of another full scale Korean conflagration.