It could be that the NK has taken over the weirwood hivemind and the whole thing has turned parasitic with the goal of attracting more human souls. NK has some super warging power where he can reanimate and control without returning souls.

Note that the Starks are buried in the crypt. There apparently was some pact a long time ago where I think the Stark bodies were to be given to the weirwoods. The breaking of the pact could be why the NK is returning.