A "folk tale" I've heard related to this is that Arthur Murray taught EVERY dance with the gentleman starting by stepping forward on his left foot. When the Fred Astaire studios were established ("stealing" some instructors from Arthur Murray), they taught Rumba starting with the side step instead of the forward step to avoid a lawsuit for "stealing" patterns from the Arthur Murray syllabus. (I don't know enough about copyright law to know if it is possible to copyright a dance pattern.) As the story goes, Fred Astaire (the studio, not the person) claimed that starting with the side step is more like the way Cubans dance—I have no idea if this was actually true. It's a good story, at least.