Ordinarily ocean bottoms are places of slow deposition of fine-grained material. Such freshly deposited rocks are soft and generate weak multiple reflections. But in polar latitudes the scouring action of glaciers removes sediment. Where erosion is taking place the freshly exposed rock is stronger and stiffer than newly forming sediments. Thus, stronger sea-floor reflections.
Continents erode and deposit at all latitudes.
However, one might speculate that on balance, continental shelves are created by deposition in low and middle latitudes, and then drift to high latitudes where they erode. While highly speculative, this theory does provide an explanation for the association of multiple reflections with polar latitudes.