In Kirby's adventure, Dedede worked FOR the
nightmare, but in Nightmare in DreamLand, he's been working
AGAINST the Nightmare. My theory is, he was tricked by a
costumed worker of the Nightmare -perhaps the NightMare
itself- to hide the Star Rod and the villian in the costume
told him where! So DeDeDee was
tricked into working for the
nightmare, he just didn't know it!
King Dedede and
Nightmare Theory