I agree with all three of those scenarios. But the third is the one. Their window of opportunity is running out. Their bombs will not last forever (no infinite shelf-life, you know) and opportunities like this don't come along every day.
They like to use big external events to divert public gaze away from internal problems, usually bashing South Korea. There will be lots of internal opposition to Kim Jong Um becoming the great successor, so seizing power quickly and effectively taking his country to war is his best hope of securing power.