There's a brand new Blogger bug with labels.
The drop down for attaching new labels to existing posts shows"Publish" where it should show "create new tag".
The Publish really publishes -- I unwittingly published some old draft posts.
I'm seeing this on only one of my blogs, it was fine on another.
The workaround is to first add the label to one post by editing the post, then apply it to the selected group.