I ran through how various Eastern European countries wound up as part of the Soviet Bloc in a previous answer I wrote here.

Czechoslovakia was a bit unique in that it was a member of the Allies that was also on fairly decent terms with the USSR (for historic geopolitical and vaguely Pan-Slavic reasons). Its brief occupation by US and Soviet forces ended rather quickly after World War II. The Czech lands in particular were already very industrial at the time, and the country's communist party was very large and had a fair degree of local popularity (unlike other countries where the local communist party more often than not was created and supported by Soviet occupying forces).

The Czechoslovak communists were the largest postwar party by vote share in the 1946 elections (but a plurality of votes - around 38% of the total, not a majority), and participated in the postwar government, notably holding the Interior Ministry and control of the police. The heavy-handed and repressive actions of the Communist Interior Minister alienated much of the country, and it was predicted that the Communists would lose significant vote shares in the upcoming May 1948 elections. The Communists basically decided (with Soviet encouragement) to plan for a seizure of power, and with the firing of significant senior non-communist police officers in the Interior Ministry provoked a constitutional crisis in February 1948, as most non-communist government ministers tendered their resignations in protest. The Czechoslovak President, Edvard Benes, facing increasing communist demonstrations and a possible Soviet intervention, accepted the resignations without keeping that government in place as a caretaker government: the government in effect was turned over to the Communist Party and their Social Democrat allies. A National Front was formed out of the latter which overwhelmingly won the 1948 elections and instituted a "people's democratic state". In the meantime, the non-communist Foreign Minister Jan Masaryk had died under mysterious circumstances in February (he fell from a window, but might have been thrown by Communist agents), and Benes himself resigned from the Presidency in June.

In short, the Czechoslovak Communists had a reasonably strong basis of support and a role in the postwar government, and then staged a coup rather than face a possible loss of power in upcoming elections. This takeover was a major shock to Western countries and significantly deepened tensions in the nascent Cold War.