apple trees

This is natural and is known as the 'June Drop' - yes I know it happens in July but that is what it is called. It is the tree's way of regulating the crop and making sure it doesn't use too much energy producing fruit.
If after this there is still a lot of fruit being formed, it is best to reduce again to a couple of apples per cluster. This prevents the tree becoming exhausted and producing only a few fruit next year. Once a tree gets into this biennial cycle, it is very difficult to get it out of it