Someone has already raised the question of ethnic Russians and other non-Baltic origin leaving these countries. This section of the population actually have a very strong incentive to leave - they are considered non-citizen and have none of the EU privileges[1].
According to the same Wikipedia article, Latvia alone started out with 715k non-citizen in 1991, and now has 290,660 as of March 2011 census. Some left, some died and some became citizen, but the drop is huge and the outflow is intentional with the aim of increasing the proportion of "native" ethnic groups.
At the same time, Russia has been taking in immigrants. The 1 million drop does not show the changes in underlying ethnic proportions. Russian politicians have not been as concerned with maintaining the ethnic composition, which they may start to look at as the riots fueled by ethnic tensions have become much more common.
In any case, pure economic speculations don't do justice to that region. It's a very complex balancing act, with nationalists on both sides willing to chop of their nose to spite their face.