There is a tag called "untagged". All of the questions have only this one tag (which has no wiki), which might suggest that there were no appropriate existing tags at the time and the asker didn't have enough reputation to propose a new tag. Given how many tags exist now, should that tag still be around?
If a tag is only used once in 6 months, the tag is removed automatically. If such a tag was the only tag on the question, it is retagged as untagged.
The way to get rid of this is to use more general tags for these questions and retag them all.