Simple question, simple answer: The Bolsheviks overthrew the Russian Provisional Government, which was a liberal-socialist coalition.

In the February Revolution (or March 1917, because Russia remained on the Julian calendar,) protesting workers and soldiers overthrew the Tsarist monarchy in Petrograd, and the Tsar abdicated on March 15. Following this, Russia’s elected Parliament, the Duma, drew up a new government to run the country in the short-term until elections could be held by the Constituent Assembly. Prince Lvov, a non-aligned liberal, headed this new Provisional Government, and most of its members came from the moderate, bourgeois liberal party, the Kadets.

However, another government existed in the capital in 1917, the Petrograd Soviet of Workers’ and Soldiers’ Deputies. Leftist revolutionaries concentrated here, including the Bolsheviks, the Mensheviks, and mostly the Socialist-Revolutionaries (SRs). The Petrograd Soviet blatantly challenged the Provisional Government’s leadership, especially because the Prov. Gov. advocated staying in the war. The Soviet called for immediate peace.

In May, after violent protests, the Prov. Gov. allowed socialists into a coalition. SR politician Alexander Kerensky became Minister of War and then in July replaced Lvov as Prime Minister. So by mid-1917 the Provisional Government was a liberal-socialist coalition. However, it had very limited real support. The Russian Army was quickly disintegrating at the front, and Kerensky’s support for the war destroyed his base at home. Furthermore, during the summer of 1917 both the Bolsheviks and the hard-right attempted putsches, and Kerensky ended up alienating both the left and right of the political spectrum. In October 1917 (the November Revolution, again, calendar difference), the Bolsheviks overthrew the largely-friendless Prov. Gov. without too much difficulty, which started the Russian Civil War.

PS: The Russian Civil War is often framed as a battle between communist Reds and monarchist Whites. But none of the major White Army factions was actually monarchist, although some of their officers certainly were. Rather the Whites claimed to be fighting for the Constituent Assembly and a democratically-elected government (although whether this was just empty talk or not is up for debate. But the Whites had no connection to the Tsarist monarchy or any intention of restoring it).