Why did the framers believe that an independent judiciary was so important?

The Founding Fathers believed that an independent judiciary was so important because they believed in the separation of powers, and didn't want either the executive branch or Congress to gain too much power and become tyrannical. 

Opposition to the nra came to a head in 1935, when the conservative supreme court declared the agency unconstitutional, claiming the nra gave the executive branch regulatory powers that belonged exclusively to congress. this decision began a series of court decisions that overturned key elements of the new deal.