Another reason was that on June 28th, 1914 Archduke Franz Ferdinand and his wife were assassinated. Archduke Franz Ferdinand was part of the Austro-Hungarian Empire. His wife Sophie was the Duchess of Hohenburg. They were both killed by Gavrilo Princip who was part of the Bosnian Serb nationalist.
Then after the assassination Garvrilo Princip was arrested. Before this Garvrlio Princip tried to kill Archduke Franz Ferdinand and his wife once before by using a bomb but missed. But the second time he shot them and killed them. There for he got arrested on the same day of the of the