During the years immediately past the Civil War in America a major shift began with regard to the silhouette of women's clothing. Where for the prior two decades the bell shape of skirts had been doing a steady expansion, the apex of what could be done with the technology to hand had been reached and a consequent deflation began. First, the skirts narrowed a bit on the front and sides, and the extra volume began to be moved to the rear, as a trained effect became fashionable. it was the precursor to the first iteration of the bustle to show itself.
My thanks to Vintage Textiles for this one.