The correct answer is: "Muslim Turks thought Christian Armenians were supporting Russia.".
Muslim Turks that constituted the majority in the Ottoman Empire, already blamed Christian Armenians for the defeats suffered in the last armed conflicts in which the empire had participated (for example, in the First Balkan War). They believed on the treachery committed by the Christian minorities.
When WWI started, both Russians and Ottomans attemped to recruit Christians who mainly lived along the Russian-Ottoman border.
In 1915, the Battle of Sarıkamış constituted the worst defeat for the Ottomans during the war, and Turks again accused the Armenians for treachery and blamed them for the defeat suffered. At this point, non-Muslims started to be demobilized and sent to labor camps. This is how the genocide started, and it would subsequently mean the systematic extermination of 1.5 million Armenians perpetrated by the Ottoman Empire.