A migrated question that does not have the tags on the migrated site (if we do not have a star-trek tag when a Star Trek question gets migrated to us) then the question is auto-tagged with untagged.
If a tag is used only on one question for 6 months, where this tag is the only tag to the question. The system will auto-delete the tag and re-tag the question with untagged. This does not bump the question. The way to avoid this is to have more than 1 tag on the question.