In 1916 Belgians took control over the area as a colony. Prior to this the Hutu and Tutsi were were both living in two kingdoms in the region, Ruanda and Urundi, with both being ruled by the same king. While they identified as different groups, intermarriage was common and there was little animosity between the groups. When the Belgians colonized the two kingdoms they chose the Tutsi, who comprised about 14% of the population, to be the ruling class. They were chosen both because this gave the Belgians a proxy to act for them in the region, and because the Belgians felt that the Tutsi were biologically superior to the Hutu.
It was therefore the Belgians who socially constructed the ethnicity of these two groups into what they became post-colonization. The labels imposed on the Tutsi for being superior caused the Hutu (who always made up the majority of the population in both the precolonial kingdoms and the Belgian colony)to become second class citizens, which they then rebelled against. This rebellion is what led to the Rwandan genocide.
Purvis, A. (1996). Roots of genocide. Time, 148(7), 57. Retrieved from Academic Search Premier database.