The main cause, the main consequence, of Nazis not disposing correctly of human waste, was the appearance of several contagious diseases that further endangered the lives of the Jews, and others, trapped within these places.
From 1940, roughly, and maybe before that, till the end of the Second World War, Germany forced Jews and other undesired groups, into settlements known as ghettos. Because of the purpose of these places, mainly torturing and eliminating the Jewish population, which the Germans hated, sanitary conditions, not to mention anything else, were less than desirable. Among the many things left unattended by the invading Germans, was the lack of attention to the disposal of human waste, which was simply thrown into the streets and garbage cans of ghettos. In the end, apart from famine, contagious diseases helped to decimate the population trapped within these horrific places.