Because NATO did not expand. Countries decided to join NATO. You may think this is the same with other words, but it isn't.
In the first version there's only two actors in the world, who can make decisions: Russia and NATO (or rather USA, for they argue NATO is basically the USA with another name). In the second version countries are free to decide who they associate with. There wasn't some empty space that NATO moved into. The countries who were there said "hey, NATO. Russia has been attacking and oppressing us for a long time and we fear they will do it again. We'd like to join."
To answer your second question: Exactly what Russias neighbors feared is the reason. Russia is unhappy they couldn't oppress their neighbors anymore since 1990 and they want to do it again. They cannot really do it to the countries that already joined NATO, so they did it to the country that was late to the party.
In the first version there's only two actors in the world, who can make decisions: Russia and NATO (or rather USA, for they argue NATO is basically the USA with another name). In the second version countries are free to decide who they associate with. There wasn't some empty space that NATO moved into. The countries who were there said "hey, NATO. Russia has been attacking and oppressing us for a long time and we fear they will do it again. We'd like to join."
To answer your second question: Exactly what Russias neighbors feared is the reason. Russia is unhappy they couldn't oppress their neighbors anymore since 1990 and they want to do it again. They cannot really do it to the countries that already joined NATO, so they did it to the country that was late to the party.