Why does ice cream make soda fizz?
Soda contains $\ce{CO2}$. Over time the gaseous $\ce{CO2}$ comes to the surface. The ice cream increases the rate of this  $\ce{CO2}$ from the soda causing increased fizzing. This increased rate is a result of the solvation of ice cream particles in the liquid. The particles serve as nucleation sites for the growth of gas bubbles. The bubbles get large quiet fast thus you observe fizzing.  

After a bit of experimentation I found the following:


Effect of adding milk &mdash; There is a bit of extra bubbling and a curdy precipitate is formed. (Couldn't dare to drink it.)
Effect of adding salt (yes ice cream does contain salt) &mdash; Lots of bubbling around the salt. After the salt settles to the bottom, bubbles collect around it become big and then come to the surface. 


Only thing left to try is sugar and vanilla essence but I ran out of soda.

So we come to know that the fizzing effect may be due to milk or salt or a combo of both.