So you say you don't see any active transactions on the system, yet you observe high rollback writes activity. But are there any writes happening at the same time in other tablespaces/datafiles?

I guess you are encountering so-called "delayed block cleanout" mechanism in Oracle. Oracle does not necessary write to database files all the necessary informations in all changed blocks after each commit, particulary after masive block changes (like your complete refresh). Large number of blocks can remain on disks marked as "dirty" and are "cleaned out" only when they are read for the first time after the change. So in fact a pure read-only aplication can cause quite extensive write activity after a mass data changes.

If you wan't to avoid this to happen during normal working hours you can simply isue a full table scan on each of your changed tables/snapshots after your refresh job is finished. This will cause the delayed block cleanout to happen immediately after the refresh.