Probably to relink the user with Pretendo, so to switch without having to backup and restore all saves.
There seems to be some way to unlink the NNID described here but it didn't work for me when I tried it:
https://gbatemp.net/threads/deleting-wii-u-user-without-nid-password.493387/post-10192667 - Also not even sure if it would work to relink a PNID after that even if it would have worked as expected, so make sure to take save backups and stuff none the less and be prepared for data loss.
Maybe
@NinStar knows more about if this method could be used to relink or if this is a bad idea to begin with.