What's that? You've never heard of the CSTO?
It's one of the latest in Russia's wishful thinking back to the Soviet times. The Commonwealth of Independent States was the figleaf when the Soviet Union lost its republics. And the Central Asian states weren't quite ready to leave the Union. Later, there were the Shanghai Cooperation Organization and the Collective Security Treaty Organization. You can probably guess that that is Russia's attempt at an answer to NATO.
About two weeks ago, Uzbekistan left the CSTO. Its president, Islam Karimov, is a dictator left over from the Soviet times. With the war on in Afghanistan, transit for ISAF and particularly the US through Central Asia has become important. Russia doesn't like the US in Central Asia. Central Asian states like leverage against both Russia and the US. So Karimov has been in and out of the would-be-Soviet alliances, which, by the way, have very weak structures, so it's easy for him to play his games.
Some interesting articles on this event. The motivations are likely a combination of what I've said, but the individual articles stress different factors.
Nathan Hamm, "Uzbekistan Exit from CSTO Reveals Limits of Russia’s Eurasian Integration Plans"
Richard Weitz, "Uzbekistan's CSTO Withdrawal Highlights Russia's Dilemma"
Alexander Golts, "The Collapsing CSTO"
Nicholas Redman, "Is Tashkent clearing the decks for the US?"
Cross-posted at The Agonist.