My understanding is that waitlists that come through within 30 days come from member cancellations.
It doesn't seem very likely that they would come from breakage inventory, since by definition, members have access to that inventory - they just compete with the CRO/WDWTC customers for it. There wouldn't be breakage inventory to send CRO if there were waitlists, and if inventory was sent to CRO under breakage procedures, members would still be able to book it (if it wasn't booked by CRO customers before the member called).
It's possible that MS could give a waitlist request available inventory from Developer-owned inventory, but I do not think that is common. MS would have to "trade" those nights for other nights in the Member Inventory. (Developers own the room nights and members are not entitled to them). That may or may not be possible. I honestly do not know the policy or procedure to do that.
Anyway, I believe that nearly all waitlist requests come from member cancellations. People' plans change.
If you are on a waitlist right now, good luck!