The oligodendrocytes makes the myelin sheath in CNS and schwann cells make it in PNS. What decides where the oligodendrocyte or Schwann cell will attach and start forming myelin sheath? Is it genetically determined? Is it random? Is there any disease associated with improper positioning of node of Ranvier?
A combination of differentiation site, chemical guidance during migration, and signaling cues form a variety of sources.
I don't know of any diseases that just mess with positioning but there are diseases that kill oligodendrocytes (like multiple sclerosis).