The first bustles were pads…

.. much like in the 18th century for “false rumps”.  The hard-edged front lines of the 1880’s made women want to soften the overall look, so the late bustles (as opposed to the 1870’s early bustles discussed already here) had to make even more visual impact than simply making the rear end look a bit bigger and softer under back flounces.

(Photos:  (left) the 1760’s false rump, used in the 1870’s 1st bustle era and the late 1870’s Natural Form Era, could no way fill out the skirts of the 1880’s (right)