Blossom drop happens when a tomato plant aborts its flowers. as in the photo below.
This happens when the air is too dry or too humid and the pollen finds it difficult to find its way to the ovary in the flower.
The way to help avoid blossom drop is to shake flowers and stems. It is also a good idea to hold something that vibrates, such as a toothbrush against the stems and flowers to simulate the affect that a bee has when it helps pollen move within a tomato plant's flower.
It is frustrating when an entire truss of flowers fails to pollinate and set fruit, so keep shaking those plants, stems and flowers!
More information about blossom drop may be found here.