What is with photographers, musicians and alleyways? You might ask. There are a few answers. (1) Alleyway walls have great character. No-one bothers to keep them in good shape. They lend a raw, urban feel to a portrait, without being distracting. One can find unusual colors, textures and if you're lucky, street art. (2) No signage. Recognizable corporate logos in the background of any portrait is quite undesirable. And it's nice that no other people are around to get in the way of the shot. (3) Shade. From the tall buildings around the alley. Good lighting for portraits. (4) Hassle-free. People are sensitive about storefronts , and permit (+ sometimes a fee) is required for shooting inside most buildings.
Why musicians specifically? I'm going to guess. There are few professions where one can wear everyday street clothes and yet be uproariously applauded. Not that rockers don't dress up. But there is something about the aura of the indie rock musician that suggests they are a part of an urban tapestry. They are not mainstream (ie, not main street), yet the city is definitely their habitat. Am I just blabbing? Probably.