What is fruit drop? Why has it been so intense this year?

Fruit drop is the process by means of which the tree decides how much energy it is going to dedicate to fruit and how much to growth. At the fruitlet stage, it eliminates a varying proportion of the fruitlets. We can then complete the process by manually thinning the groups of fruit so that the remaining fruitlets will be able to grow to a good size.
This year, the natural fruit drop was very intense: some growers were surprised to see a carpet of fruitlets on the ground under the trees. What was the reason for this?
This year, weather in the UK was very different to normal particularly in spring. Usually a tree starts growing at the end of May, but this year, the cold conditions and cold soil meant that the tree didn’t get the incentive to start growing until the second week of June. Then it realized (please forgive these verbs that seem to imply that trees have a human-like intelligence, but it’s easier to explain this way!) that it was late in its growing season (this information came from day length). So the tree started producing loads of new shoots.
New shoots need energy, just like the fruit which was also developing. This reduces the energy available for seed growth. When the seeds do not develop properly, the tree abandons the fruit around it, and it drops.
Different varieties have different thinning characteristics. James Grieve has a perfect, automatic thinning. No manual thinning is required. This year, Early Windsor ignored all the seasonal information as described above, and set a heavy crop.