He did it because Europa isn't spending the promised bnp on military spending and he is saying they need to step up their game and can't just expect for the US to come saving them every time.
Some of Trumps comments are obviously absurd, but many of them are taken out of context just like you did here.
Russia will reclaim the Baltic states if we give Putin reason to believe we won't honor our commitments. Hell, Trump's been so eager to embrace Russian propaganda he'll probably just gift them.
There is no point to wild speculation about Russia invading the Baltic states at some future date when the possibility of war with Russia exists RIGHT NOW over Syria and the allegations of cyberwarfare from Hillary Clinton and the current administration.
Trump's rhetoric is entirely focused on doing what's best for the American People, without regard to idealistic, imperialistic, abstract Neocon ideology of expanding US hegemony and influence.