Fermentation is the oxidation of organic carbohydrates under anaerobic conditions. The classic example is the fermentation of sugars into ethanol and carbon dioxide utilizing yeast for beer and wine production. During this process, CO2 is released into the air in the form of gas bubbles. But much CO2 is also dissolved in the water solution. CO2 dissolved in water produces carbonic acid (H2CO3) in small quantities according to the equation below:
H2O + CO2 --> H2CO3
Carbonic acid is a weak acid that will lower the pH slightly. So this is why the pH changes slightly during fermentation.
Another possiblity is that lactic acid fermentation has occurred. This is a less complete fermentation that converts sugars to lactic acid, another mild acid that will lower the pH.