Bubbling huh? No clue!! I've always taken my fruit (peaches, nectarines, plums) sliced it up, added sugar or honey and if you'd like spices (cinnamon for example tastes great with pears)or even flavor extracts (vanilla is great with peaches,) put it in a large glass jar with lid and let it sit for a few weeks or even months...then added a very good quality vodka,and again let sit awhile. Strain thru cheesecloth or fine strainer. You could even add grain alcohol.I've mixed up various types, and used different extracts and spices and have had some really delicious cordials. You can also GOOGLE "brandy and cordial recipes". There are a lot of them out there. Good Luck and Enjoy!!

sounds like perhaps the sugar is feeding a wild yeast, which would get you bubbles, but this is based off my making plum wine 7 years ago, it may be right, but if the container is open to air....

Might be the water is thinning it down enough to let the yeasts in the air or on the fruit start to work on the sugar. I have done a similar mix with chokecherries, 4c chokecherries, 4c sugar, 4c vodka, put it in a gallon glass jar, screw the lid on tight, shake up and put in a cool, dark place such as the basement. Shake once a week or so to mix everything up well and continue until the sugar is completely dissolved(6-8 weeks or so). Makes a very nice mixer or a wonderful addition to malted milk!--but carries quite a punch just straight.