What action did the western allies take after world war ii that caused the soviets to strengthen their hold on east germany?

The fact that the Allies were installing a Democratic government.

This depends on how long after the war we are talking about. Another reason is because the Allies (the US) started stockpiling nuclear weapons there and The Soviets wanted an ally/place close enough that if something did happen regarding the launch of those nukes, they could strike West Germany quickly