Potassium citrate in water
Bubbles need nucleation points to form. 
The growth of bubbles is limited by the surface tension of the liquid. The surface tension acts stronger on smaller bubbles and less strong on bigger bubbles.
Adding a solid salt to sparkling water means adding a lot of new nucleation points, which subsequently will lead to bubbling.
The small bubbles will combine to bigger bubbles, these bigger bubbles are not so much effected by surface tension and so even more gas is produced.
In other words, adding the salt will trigger a kind of avalanche effect, which weakens either if the dissolved CO2 gets towards used up or the salt gets dissolved completely.

It is not an reaction of dissolved potassium ions with the CO2, nor is it a reaction of dissolved citrate ions with the CO2.
What happens here is a physical reaction and not an chemical reaction.