There is little point speculating about NK reasons as it's completely closed society and we really don't know.
You can speculate weather it's a part of a power struggle, a power transfer ceremony, dissatisfied military, protest against SK maneuvers, attempt to strengthen negotiation position or a ploy to grab international attention.
In the end we don't know due to nature of that country.
And nobody claims that every single North Korean is evil. But they work for a regime that employs questionable policies.