Beck showed up on an instagram live chat with David Brown, a very old friend. Brown had played in Beck's live band on the Midnite Vultures tour (as part of the Brass Menagerie horn section), and goes farther back with Beck as friends/roommates from the early '90s.
They discussed their long friendship and stuff for about 10 minutes, and then Brown said that he used to request Beck play "Goodtime Charlie" all the time back in the day. (Why? Who knows.) So Beck tapped into his old folky vibe and played a short solo version of the song.