How exactly did NK advance in their technology so quickly all of a sudden? Not too long ago, they were struggling to get anything off the ground effectively.
There are speculations about Yuzhmash manufacturer from Ukraine. New korean rockets looked similar to rockets produced by Yuzmash. If it's true, there are two possibilities: either Yuzmash secretly sold techologies to DPRK (it existed only because of cooperation with Russia, and after 2014 it was ended as well as any money incomes for ukrainian plant), or DPRK agents just stole secrets (it would be quite easy, and DPRK agents already tried it in 2000s)