"padding in the upper back part of a skirt," 1788, of uncertain origin, perhaps from German Buschel "bunch, pad," or it might be a special use of bustle (n.1) with reference to "rustling motion."
BUSTLE. A pad stuffed with cotton, feathers, bran, &c., worn by ladies for the double purpose of giving a greater rotundity or prominence to the hips, and setting off the smallness of the waist. [Bartlett, "Dictionary of Americanisms," 1848]
Century Dictionary (1895) notes that, in addition to "improving the figure" it causes the folds of the skirt to hang gracefully and prevents the skirt from interfering with the feet in walking.