It sounds like oxygen is your problem. Well water is usually very low in oxygen to begin with and throwing it in the air might only increase the oxygen enough to aerate the surface. You probably have stratification with low oxygen in a large portion of the depth of the pond. When the wind blows hard it can force a mini turnover bringing some of the low oxygen water to the surface and trap and suffocate some of the fish. The few dead are a symptom of potential larger kills in the future. I have seen ponds with the same problem. The owner thinks that fountain of well water is aerating the pond but because it is so low in oxygen to start it is no net gain. I am guessing that your pond is fairly deep and the deep water is low in oxygen. Using diffuser aeration from the bottom will solve this problem. Be careful when starting it up because it will throw up a lot of dead water initially. If you do it in the winter you will be much safer, but in the spring or summer you will be on shaky ground. If you can get someone to check the oxygen levels it would help to get an idea of the extent of stratification. It would help to know more about the area and depth of the pond.