The kingdom of Romania is a Dictatorship, in which two Factions are facing each other: King Carol II of the Hohenzollern dynasty with his Prime Minister Ion Antonescu, and the Iron Guard of Codreanu. The latter had been executed together with several of his followers in November 1938; as Revenge, the Guard assassinated Prime Minister Armand Călinescu on September 21st of 1939. Ideologically, Nazi Germany is closer to the Guard, but currently, they still support the king because of pragmatic reasons.
In May 1940, the government signed a Pact with Berlin, about the exchange of German Weapons for Romanian oil from the Region around Ploiesti. In June, when the Wehrmacht overwhelmed France, this meant that the whole pro-French Elite was discredited.
Meanwhile, the neighboring states - the Soviet Union, the "kingdom" of Hungary, and Bulgaria - wish to regain certain lands from the (relatively) oil-rich country, which almost doubled in size thanks to World War I...
On June 20th, Molotov gave Romania an ultimatum to cede Bessarabia and northern Bukovina, 17,000 sq km. The government gave in, and on the 22nd, Georgy Zhukov's troops occupied the territory. That's why the Romanian government fell on the 28th.