Why does my soda fizz so much when making an ice cream float?
When carbonated root beer comes into contact with the ice cream, carbon dioxide bubbles are released. Likewise, the soda helps to free air bubbles trapped in the ice cream. The fat in the ice cream coats all these bubbles, protecting them and allowing them to expand to create the huge heads of foam you see on root beer floats.


https://wonderopolis.org/wonder/why-is-there-so-much-foam-in-a-root-beer-float

To expand on the first point, I believe they are referring to the ice crystals providing a nucleation site for the dissolved gases.