My dad was working for the Georgian government at the time, so I followed it pretty closely. Fact is the Russian "peacekeepers" had been breaching the terms of the ceasefire for a long time, going as far as letting Ossetian militias shell Georgian positions from within the peacekeeper's base, and also moving in more and more troops and stuff.
Then, on the night of the 7, Georgia got intel telling of a large column of armor moving in the Roki tunnel, the only connection between South Ossetia and Russia. Sakaashvillii decided to strike to try to take down the road from Roki to Tbilisi and fun time ensued. But no matter who actually shot first, the Russians had been provoking for a long time and were just waiting for an excuse to invade (hence why they had a whole army at the ready).
While the pretext was "protecting their citizens" (They had been handing out passports in the separatists provinces for month to manufacture those citizens), the real reason was to try to get Sakaashvilli, who was straining too far toward the West. It's easy imagining them wanting to punish the Estonians in the same way (The place where all the advisers used to hang out in Tbilisi was a flat called the Estonian kitchen. My dad still got a mug laying somewhere).
BTW, Ossetia and Abkhazia both had large Georgian population before the ethnic cleansing of the early 90 (Abkhazia) and 2008 (South Ossetia). Abkhaz were actually only 30% of Abkhazia's population.