Its if you put to much sugar in to that it gets to much co2 for the bottles to handle. I.E. one oz might give you 1 volume co2. 2 might give you 2 co2. 3 might 3 co2. Most bottles are safe up to 2.5. any higher and you risk creating 'bottle' bombs. Pasteurization stops the bottle at 2, while retaining the sweetness of the drink.