When you first drop the raisins in the fizzy lemonade they sink to the bottom of the glass because they are more dense than the fizzy lemonade. But the carbonated soda releases carbon dioxide bubbles and these bubbles love to attach to the rough surface of the raisins. They act like tiny floatation devices that lift the raisin to the surface of the water. This is due to an increase in buoyancy.
Once the carbon dioxide bubbles reach the surface of the lemonade they pop and the gas is released into the air. This makes the raisin lose buoyancy and fall back down to the bottom of the glass.
This continues until all of the carbon dioxide has escaped and the lemonade is flat!