The fizz comes from carbon dioxide gas, which is dissolved into the drink. When you opened the bottle, the gas began to come out slowly. Tiny popping bubbles made a fizzing sound.
When you put salt into the soda, a lot of gas was released at once. On the rough, pointy edges of the salt grains, the gas was able to form bubbles much faster than usual.
You may have seen this fast bubbling before, if you ever dropped a soda pop and then opened it right away. Why did soda spurt out?
Soda pop contains tiny bits of material. Like salt grains, they have edges and points where bubbles can form. The bits settle to the bottom of the bottle, where they have little effect.
Dropping the bottle mixes the bits throughout the drink, where they act the way salt does. As you have seen, that can release a lot of fizz all at once.
Photos by Guy Cali Associates, Inc.