It's true that basically all elected officials in SF are Democrats, but in practice there are two semi-official parties: "progressives" and "moderates." The supervisors pushing this, Aaron Peskin and Ahsha Safaí belong to the progressive wing, which recently got a majority on BoS with Rafael Mandelman taking Scott Wiener's old seat.
While pretty much united on national political issues, the two groups differ on housing policy (generally, moderates want to build more, progressives less), homelessness, and the proper attitude toward the tech industry (the progressive wing tends to blame it for many of SF's problems).