That is the lower priced line of Reese hitches. So it may not have the tightest of tolerances. It may be that the top part of the hitch has some play where it attaches to the center side to side pivot pin allowing the top to slide backward and forward slightly on that pin as you start and stop.
Unhitched, see if you can push/pull the top of the hitch and feel it move a little. Even a very small amount can "clunk" when starting and stopping. You might be able to press a plastic spacer down onto the pivot pin to close the gap, if there is one.