the sound waves are causing the salt to make those formations

Yeah I got that much. But how? Like why do they react like that?

When the sound waves are produced they are produced in a circular motion starting from the origin (imagine the ripples as you throw a pebble into a lake.) Because the amp has multiple places in which the sound can come out, it will create points where the waves come into contact with each other. The distance from the origin that the waves meet is proportional to how quickly the waves are being produced. When the waves do meet, they cancel out, because they are at the same frequency. These waves cause vibrations which move the salt, except for the positions in which the waves have been cancelled, which is why the salt remains motionless at certain spots.