They're all Eurozone now, so EU citizens can move between those countries without issue. Part of the population decline can be attributed to youth moving to central Europe for work.
Additionally, other posters have been right about ethnic Russians leaving the Baltics, which is something governments have actually been trying to encourage anyhow. The Soviets had hugely increased the number of ethnic Russians in the Baltics, nearly to the point of overtaking the native populations.
Wikipedia has a partial graph for Latvia: http://upload.wikimedia.org/wikipedia/commons/b/b1/Ethnic_gr...