Naturally the orange trees exhibit certain amount of premature falling of fruits during there growth period (may be to avoid excess load). But if fruit drop is excessive it might be due to the following reasons,

1. Due to lack of water or excess water
2. Nutrients deficiency
3. Sudden fluctuation in temperature
4. Pests and diseases
5. Excessive pruning

Its wise to water the trees deeply once a week (or more often if the weather is warm or windy) and avoid drying the soil or wilting the tree since water is the key for flower and fruit development . If possible provide mulching to reduce excess loss of water from soil. And also make sure to provide good drainage to soil.
Soil testing helps in identifying any imbalance in nutrients. Also look out for any pests like thrips, mites or diseases like canker, brown rot etc.,

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