[Schone23666]

The Baltic countries, for better or worse, sit in a strategically important area with access to the Baltic Sea. The Russians for at least since the 18th century have been involved in the area, either through "Russification", military occupation or just influencing the Baltic states to allow the use of bases and ports there.

They're relatively small countries with small militaries, and it seems that the possible thinking among some may be "Well, is it really worth it for the U.S. and NATO to possibly go to war with Russia to defend three small Baltic Republics?" Add in more disinformation courtesy of Putin along the lines of "Oh, they've always belonged to Russia," and there you go.

Besides the constant air and naval incursions (probing the wire, IMO), the Russians have already detained an Estonian official, Eston Kohver, and then these tidbits, courtesy Wikipedia:

"According to Levada Center data, in 2007, Estonia was considered an enemy of Russia by 60 percent of Russia's citizens (cf. 28% in 2006, 32% in 2005), more than any other country in the world, followed by Georgia, Latvia and United States.[111] The poll was conducted two weeks after the Bronze Soldier relocation to a military cemetery and exhumation of the bodies buried there."

"Russia often criticizes Latvia for discriminating against the Russian-speaking population.[6][7][8][9][10][11] Russia has also participated in a number of cases of complaints against Latvia in the European Court of Human Rights as a third party. These cases also dealt with the violation of the rights of the Russian minority in Latvia. Such cases included Slivenko v. Latvia, Kononov v. Latvia, Vikulov and others v. Latvia,[12] Sisojeva And Others v Latvia, and Vasilevskiy v. Latvia."

"Following Russia's military intervention in Ukraine, concerns about the geopolitical environment led Lithuania to begin preparing for a Russian invasion. In December 2014, Russia carried out a military drill in nearby Kaliningrad with 55 naval vessels and 9000 soldiers.[1] In 2015, Lithuanian Chief of Defence Jonas Vytautas Zukas announced plans to reinstate conscription, which ended in 2008, to bolster the ranks of the Lithuanian Armed Forces. The Ministry of National Defence also published a 98-page manual for citizens to prepare them for armed conflict and occupation.[2]"