What factors caused the countries of eastern europe to become satellite nations to the soviet union

Many of them had long historical relations with Russia due to their geographican proximity and were established before world war the second. They were worried about agressive German expansion.

When the second war started these nations drew closer.

After the war, in the Yalta conference countries involved: the United States, England and the URRS discussed how the world will look like. Stalin insisted on having a protagonic role in the reconstruction of eastern Europe specially in Poland and he promised to honor free elections.

The Allies agreed to Stalin's plan, unaware of his true intentions. With Soviet troops still occupying these Eastern European countries, Moscow sent in agents with instructions to rig elections to ensure that pro-Soviet communist governments won. These countries would look independent, but would be totally tied to the Soviet Union.