The reaction between Pop Rocks and soda is caused by the presence of carbon dioxide gas. When the Pop Rocks are dropped into the soda, the sugar in the soda reacts with the baking soda in the Pop Rocks to produce carbon dioxide gas. This gas causes the bubbles that you see in the soda. If there is too much gas, the balloon can become over-inflated and burst.
To prevent the balloon from bursting, you can use a larger balloon or you can add less Pop Rocks to the soda. You can also try using a different type of soda, as some sodas may produce more gas than others.