To maximize the psychological impact, they usually hit it when anxiety in S. Korea dies down and things appear to go back to normal. Then they hit again, preferably the place others may not have expected, but not necessarily so.
They had some good results after Cheonan's sinking. Non-stop belligerent posturing ignited fear among some S. Korean electorate and pro-North opposition benefited. They figure that the right level of military provocation would create enough fear among S. Korean population but short of exploding outrage, so that SK population would rather succumb to N. Korea's will, abandoning tough policy against N. Korea, and starting to accommodate N. Korea's NLL agenda.
This time around, outrage won over fear. So far. They could figure the more deadly provocation could change the balance to their advantage. This is to maximize the fear among SK population that they could lose everything they have while giving some room for renewing their vain hope that they may avoid more disaster by placating N. Korea again.
In the end, N. Korea wants Finlandization of S. Korea
I suspect they (the Reds) will start shelling somewhere, probably initially shore to DPRK territorial waters, or within DPRK territorial waters, to make some noise. One thing is for sure, keep one's eye on the little fat, pot-bellied pig one and Junior. If they go to deep ground, either in Pyongyang under KWP headquarters in one of the bunkers, or worse yet, up near Paektu, they could be readying for the final act.
Maybe all these bogus "on the spot guidance" news stories for the last six days are just that..."bogus". Who is to say that he really did visit those various places and on those various days. They could well be going down below at this time.