The deeper reason is that the Baltics are in a security conundrum. Russia is inherently insecure, and Europe doesn't know where Europe ends. For security - let's compare the Russian Federation and the US.
Russia is the largest country on earth (twice size of US) yet it has half the US population.
The US has two neighboring states and two oceans that border it (with strongest navy in the world). Russia has 14 neighboring states and several neighbor areas in frozen conflict. Russia has no oceans separating its adversaries.
In terms of history, Ukraine (and Belarus) have been the routes of choice for military invasions into Moscow.
As a result of this security situation, Russia (and the Soviet Union before it, and the Tsarists before them, ...) relies on having a neutral periphery as a buffer zone. Ukraine declared itself neutral in its constitution at the formation of the country as it split from the Soviet Union. In 2019 Ukraine stripped neutrality from its constitution and wrote in pursuit of NATO membership.
For Europe not knowing where it ends, just look at EU membership, potential membership, and statements evolving over the past several decades from officials about where Europe ends.
The proximal cause of the war, literally the day-by-day timeline leading up to the conflict, was the Biden Administration threatening to pull Ukraine in NATO, Russia stated this was a red line and that it would invade and destroy Ukraine rather than let that happen, and then Russia and NATO diplomacy failed to resolve the issue.
This is clearly what the war is about: it is what led day by day up to the conflict, Ukraine's president offered to withdraw NATO ambitions as the invasion began to passify it, the Istanbul Peace Agreements early in the war centered on resolving the topic, and the peace discussions now have centered on it.