All quotes and information from "Smoldering Feud, Then War---Tensions at Obscure Border Led to Georgia Russia Clash," The Wall Street Journal, Saturday/Sunday, August 16-17, 2008, page 1.
Tension in Georgia has been ongoing since the 1990s. Very quickly after Georgia became independent of the Soviet Union, South Ossetia and Abkhazia fought a war to try to secede from the country. They defeated the Georgian forces with the help of military aid and volunteers from Russia and effectively became "unrecognized Russian protectorates."
According to this article in the Wall Street Journal, two factors pushed tensions higher this year. First, Georgia moved toward joining NATO in the face of intense Russian objections. Second, the countries of the West, ignoring strong Russian opposition, recognized the independence of Kosovo---formally a province of Yugoslavia