Employment structures change over time. Countries in the early stage of development usually have a high percentage of the population in primary employment. This is because most people are engaged in agricultural activities. As a country begins to develop an industrial base there is an increase in the secondary sector. An increase in machinery on farms means fewer people are needed.
People tend to migrate to urban areas to get jos in factories.
When a country becomes more economically developed there is a greater demand for services such as education, health care and tourism. Therfore the tertiary sector undergoes growth. By this time computers, machinery and robots replace people in the secondary sector hence the decrease in secondary jobs.