Why does pouring carbonated beverages onto fruit cause them to fizz?
All carbonated drinks are oversaturated solutions of $\ce{CO2(g)}$. Pouring over fruits lead to contact with many residual air bubbles, which serve as  fizzing places for $\ce{CO2(aq) -&gt; CO2(g)}$. 

It is more or less the same as the ( previously already opened ) drink shaking, where violent fizzing occurs as well.

It is a similar process activation, like if you bend the activation chaff in a pocket heater with molten $\ce{CH3COONa \cdot 3 H2O}$, or if you pour overcooled water from a freezer ( see YouTube ) and watch fast ice growing.