I think that the reason why some places were considered "hot" might be because the Cold War underscored an overall time period of intense change. With the ending of the Second World War and the gradual disestablishment of European Colonialism, new nations emerged. These particular nations sought to create their own political destiny and carve out their own political image. In many ways, some of this involved whether or not to embrace Western capitalism and democratic orders, perceived as holdovers from the West or the former controlling regimes, or whether to embrace a new conception such as the Marxist or Communist vision. While these nations sought to explore new aspects of their political identity, they were doing so under the guise of the Cold War. Both superpowers subscribed to the "domino theory" which argued that if one nation in a region moved to one side of the political spectrum, others would follow. This meant that an emergent nation which sought to explore its own political identity did so under the guise of both superpowers seeking to control the world with its own ideology. In this light, "hot" areas emerged because these nations ended up playing out the larger debate held by the superpowers in the Cold War.
I am not sure exactly how you mean this question.
Are you asking why they were called "hot?" If so, the reason is that these were places where tensions between the Soviet Bloc and the US would flare up -- places where they could or did erupt into "hot wars" as opposed to the Cold War.
Are you asking why some places became "hot?" If so, a major reason is because some of these places bordered on communist countries and the communists were trying to expand. This is the case in Korea and Vietnam. Another major reason is that some of the hot spots were in areas that were close to the United States. This is the case in, for example, Cuba and Nicaragua. Others were in places that were important for their resources (like the Congo/Zaire) or for their location (like Egypt on the Suez Canal).