Why does madison fear "a gradual concentration of the several powers in the same department" of government?

James Madison was a Federalist who wanted to ensure institutions kept people on check in regards to their ambitions as well as wanted to ensure that not one branch of government had power over the other. He wanted to protect the people in case the government ever tried to control the people who created it. He believed that giving one part of the government more power than another could lead to such branch taking advantage of the other branches as well as the people.