That is a recent development, more to do with the realities of national political campaigns.Since he's seeking that party's nomination to run for prez, his claim of being an independent seems weak.
Cynic that I am, I believe that the DNC let him in for two main reasons. He maintains the appearance of a campaign for the Dem nomination, without the electability to present an actual threat to her coronation. And he is so far left that she appears center right by comparison, which she is (despite efforts to paint her as a leftist).
Tom