That's a very strong cluster of population loss in Eastern Europe and the Baltics.
It's probably for multiple reasons, but ability to travel in the EU westward toward greater opportunity seems like a big one.
Latvia and Lithuania surprise me though I thought these were more similar to western and northern Europe economically. Perhaps their proximity to the border of Russia influences things.
Latvia has a large Russian population that was mostly not given citizenship[1]. What you see in the demographics is that the ethnic Latvian population is in slow decline, and it's _mostly_ this ethnically Russian population that causes the steep decline, down 50% since 1990.[2]
When they joined the EU there was a lot of closures of existing heavy industry in those countries - some say they were paid by EU members to avoid potential competition. No reason they would look like north europe economically.
In the 10s they worsened their stance with their larger neighbours Russia and Belarus which diminished their value as transit countries. And now Lithuania is also quarreling with China[0].
At the same time, Finland kept it prominence as a transit country to Russia and continues to do so even today[1].
From what I know many migrated to Poland after 2014 maidan. Simple google query yields many articles about numbers, I won't link here since I don't know about credible source I would feel comfortable to share. But from my observations during past few years I must say there must have been many, I'd risk to say buildings industry was able to grow because of inflow of Ukrainian workers. This helped post-February since many families already had somebody here so they could join. And also Polish bad historical encounters with Russians led to Polish society rising to help as many Ukrainians as possible, many school gyms and other public places were converted to host Ukrainian refugees, similar with some companies who donated their office spaces. There was a time all LinkedIn updates on my board were from such companies and from individual people who were organizing to help.
It's probably for multiple reasons, but ability to travel in the EU westward toward greater opportunity seems like a big one.
Latvia and Lithuania surprise me though I thought these were more similar to western and northern Europe economically. Perhaps their proximity to the border of Russia influences things.