I've heard several theories and Wikipedia has even more, is there a consensus amongst the relevant academic experts?
Alan Dundes, in his article "Towards a Metaphorical Reading of 'Break a Leg': A Note on Folklore of the Stage," in Western Folklore 53.1 (Jan. 1994), explores the origins of the phrase and the tradition of saying "break a leg" as a wish of good luck to an actor before a performance. Dundes explains that the expression has its roots in a similar German phrase: "Hals- und Beinbruch," meaning "break your neck and leg." Citing The Harper Dictionary of Contemporary Usage by William and Mary Morris (1975), Dundes points to historical evidence that suggests the expression came out of the Yiddish theatre tradition specfically. Dundes goes on to posit that the well-wishing tradition became attached to the theatre because the actors' bows at the end of a successful show look as if the actor is "breaking" the neck or, in the case of a curtsy or bent knee bow, "breaking" their leg (87). To support this assertion, Dundes points to a letter published in The Stage and Television Today (1992) in which Estelle Couplan explains that the original meaning of "break a leg" was to tell an actor "take a bow."