How did nubia become the independent african state that emerged at the end of the second millennium b. c.e.?

Nubia benefited from the disintegration of the New Egypt kingdom to become Kush

Nubia was once a tributary of Egypt and was the most important lifeline of the  kingdom serving as the source of gold and leopard skin. However, around 2000 b.c.e the disintegration of the vast empire that trans versed two continents,benefited Kush to the extent that Egypt itself also became a tributary of Kush.