As I understand it, wine (and therefore also wine vinegar) is produced by stirring the mixture very frequently. That encourages yeast to form. Yeast are tiny organisms that won't be disturbed by stirring. Mold on the other hand likes to form large structures that get broken up when you stir. Therefore, if you don't stir, you get mold rather than yeast, and mold doesn't seem to produce the same alcohols that yeast does. And it creates a particular taste of it's own, which most people don't like in their wine, so they avoid it by stirring the mixture.