Could be one or several of many different factors. Alot of peaches will try to naturally thin themselves, or the grower has to thin them themselves so they dont become overloaded and break and so they will grow full size fruit. Bell of Georgia is one that is bad to thin itself heavily. Not enough water can do it, pests can do it, Sevin can do it, alot more factors can do it so it is difficult to say without knowing entire history and being right on hand to see. Calcium nitrate applied in dormant season has been shown to suppress some of that natural dropping.