Sometimes, cool things just happen for no particular reason. Just go with it. St. Vincent, for instance, is in between promo cycles; Daddy’s Home, her last album, came out two years ago. Every once in a while, though, Annie Clark will pop up on late-night TV with a sick-ass cover song. Last year, she did Fleetwood Mac’s “Dreams” with the Colbert house band. And last night, she did Portishead’s “Glory Box” with the Roots and a huge string section.