Why was Bulgaria at war with both the Allied and the Axis at the end of WWII?

Question

Bulgaria entered the WWII after signing the The Tripartite Pact (1 March 1941) which subsequently resulted in a long list¹ of war declarations, unilaterally or mutually initiated, against Western Allies countries. However, there is a very interesting period of about two months: September 8^th - October 28^th 1944 through which Bulgaria de jure was at war with all the countries from both the Allies and the Axis.

What events led to that strange foreign policy situation?

Is there any other known example of a country at war with the "whole world"?

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^{1. List of WWII declarations involving Bulgaria:
1941-04-06 Bulgaria declares war on Yugoslavia.
1941-12-12 Bulgaria declares war on United States, United Kingdom.
1941-12-13 New Zealand, South Africa declare war on Bulgaria.
1941-12-19 Nicaragua declares war on Bulgaria.
1942-01-06 Australia declares war on Bulgaria.
1943-04-02 Bolivia declares war on Bulgaria.
1944-09-05 Soviet Union declares war on Bulgaria.
1944-09-08 Bulgaria declares war on Germany.}

Answer 1

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.

As an Axis Power (1941-1944)

.... 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. ....