Here is a brief synopsis from a paper I wrote last year:

Although Washington was caught off guard by the sudden rupture in Yugoslav-Soviet relations, tensions between Tito and Joseph Stalin had begun almost immediately following the former’s 1937 selection as the head of the Communist Party of Yugoslavia. The outbreak of World War II and the German invasion of Yugoslavia saw the opening of a gulf between the two countries as Tito and his communist partisans began to defy the wishes and orders of Stalin. For the sake of its own survival the Soviet Union had become allies with the Western capitalist powers of the United States and Great Britain, however the issues of Yugoslavia’s own communist revolution were more important to Tito than the maintenance of the Anglo-American-Russian alliance. As a result, the Yugoslav national struggle for liberation against the German occupiers was transformed by Tito’s communist partisans into a civil war for control of Yugoslavia. In the pursuit of this goal Tito openly defied Stalin. This can be seen in an interaction where the Soviet leader asked Tito what would be done if British troops landed in the Balkan Peninsula to which Tito answered, “We should offer determined resistance.” While such a landing would have relieved pressure on the Soviet Union and could possibly have helped expedite the liberation of Yugoslavia, Tito was opposed because the presence of British troops in the Balkans “would have imperiled his position of power.” The Yugoslav partisan leadership also became disillusioned at the barbaric actions of Red Army troops in Belgrade and wider Yugoslavia. When Tito’s deputy Milovan Djilas protested to Moscow regarding the “unworthy behavior” of Red Army troops Stalin was infuriated.
This testy relationship intensified in the years immediately following the war as Stalin refused to back Tito against the British on the issue of Trieste. Moscow additionally opposed Yugoslavia’s introduction of an extremely ambitious Five-Year Plan in 1947 as well the Tito’s demand that the Yugoslav army be equipped as an independent force. Tensions continued to mount as Stalin “categorically asked the Yugoslavs to liquidate” their support of communist Greek rebels. Additionally Tito, without first consulting Stalin, proposed the creation of a Yugoslav-Bulgarian Federation. Furious at not being consulted, Stalin met with the Yugoslav Foreign Minister, Edvard Kardelj on February 10, 1948 and forced him to sign an agreement mandating Soviet consultation in all Yugoslav foreign affairs. In response, Belgrade requested that Soviet advisors in Yugoslavia have their pay reduced to the level of their Yugoslav counterparts; Russia flatly refused and withdrew their military advisors on March 18, 1948 and their civilian advisors the following day. An exchange of letters between Tito and Stalin failed to quell their escalating dispute and on June 28, 1948 members of the Cominform, voted to expel the Communist Party of Yugoslavia.