Does this happen to every single fruit on the tree? Many trees go through 'summer drop' a natural self-thinning process.

I'm also thinking 'Pear Midge' which causes the fruitlets to blacken and fall off. If so, you would find maggots inside the fallen fruit. They pupate in the soil where they drop, and emerge as adults the following spring. Removing and destroying blackened fruit before they fall will break the cycle.