The "homeworld" of the Sith has been said to be both Korriban and Ziost...isn't it possible that Morriband is just another planet in that system that the Sith may have migrated to/from?
EDIT: According to the preview for Clone Wars season 6:
Guided by the Force, Yoda travels to the heart of the galaxy. There, on the planet where he finds the origin of the Force, he must face difficult trials. Only upon passing them will the Sages deem him worthy of learning the deepest mysteries of the Force....