|Why would most of the orange flowers fall on to the ground just when they are growing into oranges? What could have caused that?|
|Without more detail on how the tree is cared for it's hard to say, but here's some info that might help. Citrus is a heavy feeder and needs regular fertilizer applications. It's recommended to apply the total annual amount of nitrogen required in 3 applications: in Jan/Feb; Apr/May; and Aug-Sept. Requirements vary according to the age of the tree.
Citrus fruit drop is actually a natural thinning process. Citrus trees thrive on consistent moisture. That may have been a factor in the fruit drop if the tree was stressed. Apply enough water slowly and deeply to thoroughly soak the root mass. Don't sprinkle lightly, which allows salts to accumulate and burn the roots. How often to water depends on your soil characteristics, rainfall and weather, but a rule of thumb is about once every 3-4 weeks in winter and once every 10-14 days in summer. Use a soil probe to help determine how often to apply water (such as a pointed stick or metal rod) and poke it into the soil. It will move easily through moist soil and stop at dry soil. As trees mature, be sure to water out to their canopy or drip line, where new roots are growing. To keep water from evaporating too quickly, mulch over the root area. This will also help suppress weeds.
Finally, some years the conditions are right to just produce more fruit than other years. Good luck with your citrus!