I know that these questions all had tags on the old site (it was required there too), but many do not have tags anymore and are tagged untagged. Why weren't the tags carried over completely?
When a question is migrated in, any tags that do not exist on the destination site are stripped from the question. This is done to prevent tags that shouldn't exist being created through a migration backdoor.
Questions that end up without any tags as a result of the migration logic are tagged with untagged so that the community can review them.