In World War II, why did Japan switch from being a monarchy (led by Hirohito) to a dictatorship (led by Hideki Tojo)?

There was an Emperor change in 1926 and a gradual political change over the next decade. The formal structure of the government did not change from the Constitution of 1889 but the role of the Emperor changed to give him more absolute power. Leftist and democratic elements were suppressed. The "Kokutai" or "body of the State", the culture of government changed considerably. The consensus based nature of Japanese decision making often made individual contributions difficult to determine.

Japan had a number of foreign policy failures during this time (London Naval Treaty, League of Nations departure, The Japanese Exclusion Act passed in the USA) that left Japan more isolated Internationally. This helped give rise to a Nationalist movement to establish an oligarchy around the Emperor to govern.
The Army conquered Manchuria in 1931. The National Diet was controlled by Army personnel and the supported the departure from the League of Nations in response to criticism from the League.

There were a number of violent domestic incidents. A Prime Minister was shot in 1930, another Prime Minister was assassinated in 1932. On February 26, 1936 a large group of young officers and Nationalists entered central Tokyo and hunted down and killed a number of prominent politicians and attempted a coup. They failed to find the Prime Minister or gain the support of the Emperor. The possibility of violent attack from Nationalist officers weight heavily on all politicians.

This led to the establishment of a de facto cabinet based system of government under the Emperor (The Emperor attended Cabinet meetings) which placed the Japanese Army in the drivers seat. This continued until the end of World War 2.

https://en.wikipedia.org/wiki/February_26_Incident
https://en.wikipedia.org/wiki/Sh%C5%8Dwa_period