I have seen the following speculation, and have no idea if it is true.
1. We dropped something very heavy on the mountain where the tests were being conducted, OR some other direct action happened. Occams razor probably dictates that the Norks just demo'd their own mountain accidentally.
2. Kim was being held hostage by his own military until very recently, and some sort of direct or indirect action took place, possibly with Chinese support, that resulted in easing up on that leverage.