What causes the water to backflow from the little silver thing on the back of the sink for the dishwasher

That is a syphon break. It has a rubber flapper inside. When the dishwasher drains, the water pushes that flapper up to block the opening on top. When the dishwasher stops draining and water stops the flapper drops back down to allow air into the drain line to stop the water from syphoning back into the dishwasher. If it is leaking then that flapper is not sealing and the syphon break needs to be replaced.