What has been the most common reason for one party losing control of the national government

The answer is: The country suffers from severe economic problems

During severe economic problems, a large of  portion of people would exprience either unemployment or a decrease in their net salaries.

When this happen, people would start to blame whoever in power for the economic problems. If the government is controlled by a certain party, that party would gradually lose voters' support and unable to secure position in the government body.

the boundarys like convergent divergent and transform it causes subduction

the secretary of state, appointed by the president with the advice and consent of the senate, is the president's chief foreign affairs adviser. the secretary carries out the president's foreign policies through the state department and the foreign service of the united states.