Because it was Germany who assinated the heir to Austria-Hungary right?
This was an existing conspiracy theory, actually. Several people believed that Germany was behind the assassination, including Max Hohenberg (Franz Ferdinand's son).
The Serbian Government allowed for the Black Hand terrorist organization to grow more powerful until they assasinated the heir to the Austro-Hungarian Empire in what is almost certainly the greatest (in a negative sense obviously) act of terrorism ever committed.
The greatest act of terrorism ever committed? I'd say a shooting with two victims wouldn't even be in the "top" 100 acts of terrorism ever committed.
Never mind how many other countries at the time were also engaged in undermining their neighbors (sometimes in ways more severe than anything the "Black Hand" could have dreamed of).