The answer is Germany felt threatened by Russia’s mobilization of troops
Germany always had the worries, that if they entered the armed conflict, they would have to defend or sustain a war effort with an ever increasing Russian Empire. They also had to respond to back up the Austrians which, were close allies and neighbors. When Russian troops got close to its allied: Austria-Hungary, the mobilization was seen as a threat, and lead to the formal declaration of war. Germany declared war on the Russian Empire on the 1st of August 1914. The mobilization of Russian troops was an initial response to the declaration of war from Austria on Serbia, which had begun the so-called "ignition point" that eventually led to World War I.
