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That time frame, from Sept. 8 to October 28 1944, corresponds to the time period in which Nazi forces were driven out of Bulgaria by the Red Army during the Jassy-Kishinev Offensive following the Destruction of Army Group Center in Operation Bagration. The consequent replacement of a pro-Nazi government by a pro-Soviet one would seem quite sufficient explanation.
.... On 5 September, the Soviet Union declared war on Bulgaria and three days later the Soviets crossed the border and occupied the north-eastern part of Bulgaria along with the key port cities of Varna and Burgas by the next day. The Bulgarian Army did not offer resistance by an order of the government. On 8 September, the Bulgarians changed sides and joined the Soviet Union in its war against Nazi Germany.
As an Allied Power (1944-1945)
Garrison detachments, led by Zveno officers, overthrew the government on the eve of 9 September, after taking strategic points in Sofia and arresting government ministers. A new government of the Fatherland Front was appointed on 9 September with Kimon Georgiev as prime minister. War was declared on Germany and its allies at once and the weak divisions sent by the Axis Powers to invade Bulgaria were easily driven back. ....