There are several factors that caused the Balkans conflict in the years previous to WWI, but the main ones are: the weakness of the Ottoman Empire; increasing nationalism in the region; religious tensions; and strategic interests of the Great Powers in controlling the region.
Ottoman weakness: during the second half of the 19th century the Ottoman empire saw a decline in its military strength, which led it to be defeated by the Russian empire in the Russo-Turkish war in 1877-1878. There were also a series of internal attempts in order to modernize the empire politically, economically, and institutionally which drastically failed. Hence, the Ottoman Empire was unable to govern and to reform itself. For these reasons, it was known as the "sick man of Europe", and the other powers knew that this weakness could alter the balance of power in the Balkans.
Nationalism: the Balkans is a region of multiple ethnic and national identities, like Bulgarians, Serbian, Croatians, Bosnian, among many others, which were dominated by the Ottoman Empire for centuries. With the rising idea of nationalism in the 19th century, many of this national communities wanted to be independent and form a Nation-State. Serbia was indeed an independent country, but wanted to expand its frontiers to every region where there were Serbians, like Bosnia-Herzegovina -that was an Ottoman province until it was taken by Austria-Hungary in 1908-. On the other hand, Greeks wanted Macedonia and Crete, and Bulgarians wanted Thrace. These three countries united against the Ottoman Empire in the first Balkan War. After this war, Bulgarians faced against Greeks and Serbians for their common rivalries on the control of the territory of Macedonia and Thrace.
Religious tension: to the national and ethnic diversity we must add the religious diversity. The Ottoman Empire was an Islamic Empire, while the Balkan peoples were Orthodox Christian (Greeks, Serbians, Bulgarians), Catholic (Croatians) and Muslim (Albanians, Bosnian). Christian communities wanted to be independent of an Empire that didn't hold their religion.
Strategic interests of the Great Powers: Austria-Hungary was interested in the survival of the Ottoman Empire since it worked as a counterweight to Serbian nationalism. Russia was a big ally of the Slavic Balkan countries, Bulgarians and Serbians -pan-Slavic policy-, and wanted to have access to the Mediterranean Sea through this region. The Prussian Empire wanted to keep the integrity of the Ottoman Empire and then turn it into a Prussian colony. France and Britain wanted to strengthen their position in the Eastern Mediterranean.
