I had no idea.
Georgia has restarted fighting in South Ossetia, a breakaway republic discreetly backed by Russia. At the time of writing, based on reports in the Russian news media, the rebel capital of Tskhinvali has been captured... and levelled. Georgian forces have announced an imminent three-hour ceasefire to allow refugees to leave the warzone through a safe corridor.
Georgia's casus belli, as far as I can tell, is the bombing of Georgian towns by military aircraft originating in South Ossetia. It and Abkhazia have been a source of provocation for Georgia for months now, and there has been some convincing evidence that the Russian military was actively involved. (A Georgian aerial drone was shot down by a MiG-29, and neither the Abkhaz forces nor the Russian peacekeepers within Abkhazia proper have that type of aircraft.) Georgia itself has been the object of much of Russia's ire in the last couple of years, including an economic blocade and persecution of ethnic Georgians living in Russia.
There has been a scarily plausible opinion going around that as the Beijing Olympics kick off and the world's attention is elsewhere, Russia would make a move. Georgia was visibly spooked by the lack of support from the West, including Germany's block of NATO membership for the country.
It seems that Georgia has considered a military aggression on the part of Russia and/or its rebel allies inevitable, and has decided to make the first move.