What is meant by the phrase “ A sandwich sparked WWI”?

World War I took place between 1914 and 1918. It was thought that the war would only last a few days, but when it stretched over 4 years, the implications of this war became more apparent.
It was the first of its kind, regarding mortalities, countries involved and the changing landscape of the world.
Context of the Times
The Ottoman Empire had been weakening, and this caused instability in the Balkans; in 1878 Austria-Hungary agreed to look after Bosnia. Serbia felt that Bosnia should unite with Serbia to bring the Serbian people together. At the same time, Russia was worried about Austria’s growing strength.

By 1908 the Ottoman Empire was regaining strength, Austria Hungary was reluctant to give up Bosnia, and there was fear of the growth in Serb nationalism.
After the Balkan Wars of 1912 and 1913, Serbia had DOUBLED in size. Nationalism among Serbian people soared!
Austria-Hungary was not happy with the new, bigger Serbia because there was concern that the Slavs living in the Empire would want to break free and live in Serbia.
Black Hand Gang

Where is the statements?