I got different responses, often conflicting ones: It could have been from excess water as well as lack of water. While June drop may be associated with heat, even fruits which formed later in the fall dropped down in large numbers. Here are all the suggestions I was given, none has worked:
1- Lack of water
2- Excess water: I have tried a wide range of water supply.
3- Excess fruits - natural culling, so-to-speak. It should never end with five or six fruits remaining on the tree till maturity.
4- Hot weather: Even late fruits have fallen in large numbers.
5- Lack of nutrients - I have added humus & compost and supplemented with fruit trees fertilizer. I used slow-release pellets to avoid over-fertilizing.
6- 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 Mediterranen. It is zone 11, so I have no frost. Summers are hot. Main heat waves occur between July and August.