Sometimes, not all the time, your shadow was cast on the water just enough that it spook the fish and ruined that stealthy approach that had you on your hands and knees. It should also be noted, that this commandment, while applicable to any water is even more so to small streams and creeks. Think about the makeup of a small creek versus a larger river. Also, take into account the life purpose of all fish and how that plays into being so skittish.
First, a fish has three primary purposes in their lives: eat, reproduce, and don't get killed during that process. Every single instinct that a fish has is tied to one of those three purposes. What a fast, unnatural shadow on the surface water generally means death, and remember, one purposes of a fish's life is to avoid getting killed. Whether it's a hawk attacking from above or a bear's paw scooping from the side, fish avoid quickly moving, unnatural shadows. On a tangent though, fish generally love hiding in shadows cast by naturally occurring landforms, a large tree or bank-these shadows are constant and move only with the sun and generally lay on the surface of the water for great lengths of time.
Even on large rivers the best practice is to keep yourself and your shadow as far away from the view of the fish as you possibly can! On small creeks this is paramount if you want to catch a fish! As always, Good Luck and Guid Luck!