It's origins have been debated...an analogy to the bow performers give at the conclusion of a show, an reversal of the "good luck" phrase (which is actually considered to bring bad luck in theater circles), the tradition of Greeks stomping vice applauding (for a really good show, you'd get them to stomp so much they would break a leg!).
It's true origins will likely never be known for sure, the phrase having been in use so long it has simply become second nature.
However, some people seem to take the phrase quite literally.
Now, rest assured that it takes more than a broken leg to keep Suzanne down. After a short absence and a bit of medical care, Suzanne is back (crutches, cast, and all). Mary Lyle kindly stepped in for Suzanne during her short absence, but Suzanne could not be kept away, certainly not by a broken bone.
Suzanne, we'd like to remind you that the phrase "break a leg" is a traditional one...NOT to be taken literally!
("Steel Magnolias" will debut on October 2nd at 8:00 PM, with follow-on shows October 3rd at 8:00 PM, October 4th at 2:00 PM, and October 9th & 10th at 8:00 PM.)