Grapes picked, denuded from stems, put through juicer, strained through fine cloth into bucket. Yeast growth noticed so removed from top of juice, restrained through fine clothe into bottles. Bubbles noticed in bottles and within 24 hours the corks on the bottles are popping. in other years I have juice at this stage but this year the process has sped up. No other ingredients added, just the grape juice. What is happening please?
Unfortunately you have figured out how to make wine. There isn't probably any way you can stop it short of sterile filtration, heavy use of potassium metabisulfites or freezing. You can let it run it's course and you'll have some wine or next time just freeze it and drink it when you need. The grape juice you see in the stores have been sterile filtered and chemicals added to stop fermentation. Good luck next time!