Well I know a little bit about the general political situation there. Vladimir Voronin, the leader of the Communists is an ethnic Russian, but the politics of his party are not really pro-Russian. The party is actually mildly pro-EU and wants to join the EU eventually, for example it was going to sign a peace agreement with its break-away region Transnistria in 2004 or thereabouts after Russia brokered the deal, but they backed away from it at the last moment at the urging of the EU. So basicaly they sacrificed their relations with Russia for their relations with the EU.
But it could be that the Communists have been betrayed now by the West for not being sufficiently pro-West. This isn`t really anything new. For example in Georgia Shevarnadze was overthrown in the Rose revolution albeit he was moderatly pro-Western. For being seen as innefficient and not quite pro-Western enough.