I got different responses, often conflicting ones: It could have been from excess water as well as lack of water. While the June drop may be associated with heat, even fruits that formed later in the fall dropped down in large numbers. Here are all the suggestions I was given, none has worked:
Lack of water
Excess water: I have tried a wide range of water supplies.
Excess fruits - natural culling, so-to-speak. It should never end with five or six fruits remaining on the tree till maturity.
Hot weather: Even late fruits have fallen in large numbers.
Lack of nutrients - I have added humus & compost and supplemented them with fruit trees fertilizer. I used slow-release pellets to avoid over-fertilizing.
Fruit fly: No maggots were observed inside the fruit.
The drop process is quite fast. The fruits are still green and may suddenly turn soft and partly yellow and drop overnight. I followed up the process by keeping only the green fruits and watched how the ground is covered with fruit the next morning.
My climate is Mediterranean. It is zone 11, so I have no frost. Summers are hot. Main heat waves occur between July and August.
EDIT : Update on experimenting with fruit drop...
Thanks for the feedbacks I have been receiving. Last summer, I did an experiment. Based on the the climate in its country of origin (Southern China), which is a tropical climate, I decided to spread the same dosage of water more evenly and increase it just slightly. Rather than providing water twice a week, I administered a five times a week regime. This includes the regular twice a week "major irrigation" (same dosage and with the rest of the trees), plus half the dosage between two consecutive "major" irrigations. This should keep an evenly moist soil throughout the fruiting period, all without overwatering.
Gladly, I have prevented fruit drop. Rarely any fruit has ever dropped. To be on the safe side, I also used the fruit fly pheromon traps. By the end of the season, the traps were full of flies. I suppose that most fruit drop occurred because of irrigation issues, since there were no fruit-fly larvae when I had the massive fruit drop. Still, the two approaches combined has improved my yield and the fruit flies were not the only ones to enjoy the fruits...