Because Saakashvili thought he had a green light from the US & the West. The training, arms, & NATO-talk that Georgia had been receiving made it feel invincible, it seems.
This is just from a soldier, but I get the sense that the Georgian leadership was thinking along these lines as well:
http://www.nytimes.com/2008/08/11/world/eu...amp;oref=slogin
Also, when Bush et al supported the secession of Kosovo from Serbia, Putin vowed to provide similar support for the irredentist, pro-Russian areas of Georgia.