They were pretty much between a rock and a hard place. If they failed to attack, their national sovereignty and unity were at stake. They also thought (mistakenly) that they would be able to keep the Russians from interfering by blocking the tunnel between North Ossetia (part of Russia) and South Ossetia (nominally part of Georgia). Based on the (incomplete/incorrect) information they had, this was a reasonable assumption to make.

What they didn't know was that the Russians had already moved several brigades through the tunnel and controlled both sides of it before the fighting in South Ossetia started. But, hey, maybe it really was pure coincidence that those two things happened in that order, in rapid succession.