Why did the Soviets decide to blockade West Berlin in June 1948 and then to end their action in May 1949?
The Berlin blockade was implemented because the Soviets did not want the West to move West Germany towards an existence as a united and independent country. The specific action that caused the blockade was the West's creation of a new currency that would be used by West Germany. It was because of the introduction of that currency that the blockade began.
The blockade ended because the Soviets decided that it was not going to work. The Berlin Airlift had clearly managed to overcome the blockade and keep West Berlin supplied. Given that fact, there was no point in continuing the blockade so the Soviets called it off.