The immediate cause of the crisis in Berlin in those years was the Soviet Union’s attempt to blockade West Berlin and prevent the West from supplying its part of the city. The ultimate cause of the crisis was the growing suspicion and distrust between the Soviet Union and the West at this very early point in the Cold War.
After World War II, Germany had been split into four zones of occupation. The Soviet Union, the US, Britain, and France each ran a zone. Berlin was in the Soviet zone. Because it was such an important city, it too was divided into four zones. Soon after the division, the three Western allies consolidated their zones and planned to merge them into one country. The Soviet Union saw this as a great threat to them.
When the Soviets heard through their spies of this plan, they began to harass West Berlin, hoping to push the Western Allies out of the city. This eventually led to an all-out blockade of all of the land routes into the city. In response to this, the US and its allies launched an airlift, using airplanes to supply West Berlin with everything it needed over the winter of 1948 and 1949.