A citrus tree drops all its flowers before fruit develops due to stress from environmental factors like inconsistent watering (both over- and under-watering), temperature extremes (like a late spring frost), or nutrient deficiencies, especially a lack of potassium. Other causes include poor pollination, insufficient sunlight, or a young tree needing to focus on root development. The tree is essentially shedding its blossoms to conserve energy and resources when it is under stress or unable to support the development of all the fruit.
Taken from google