Why was the balkan region called the powder keg of Europe?

Prior to (and in fact since) World War I, the Balkans region of Europe was called its "Powder Keg" because of the many different competing nationalities and ethnic groups and allegiances contained in it. With major European powers only indirectly involved in the intrigues and hostilities of the Balkans, it came to be said that a match lit in that region could set fire to the "powder" and thus cause an explosion that would impact the entire continent -- and even beyond.