The first cause of the tension was the actual creation of the Soviet Union and the civil war that Russia went through during that creation. As the Soviet Union was being created, the United States government opposed the Bolsheviks. They condemned the Revolution and they sent a few troops to try to help defeat the communists. This showed that the US really did not like the idea of a communist government and it helped make the USSR suspicious of the US. After that, the US did not recognize the existence of the Soviet Union for over a decade.
The other major cause of tensions was the issue of how to split up Eastern Europe (and to some extent Asia) at the end of WWII. This caused tension because the US thought the Soviets were overly expansionist and the Soviets thought the US was trying to encircle and destroy them.