Blossom drop can be caused by

• too low or too high night temperatures, too much nitrogen,
• too much shade,
• over-watering or
• even insects, such as flower thrips.

Fertilizer with too high a nitrogen content will cause a lack of blooms or blossom drop.

There is an optimum temperature range for bloom set and production. Night temperatures below 55 degrees or above 75 degrees will often cause the blossom to drop.

A watering cycle which subjects the plant to alternating wet and dry conditions, as opposed to consistent moisture, may result in bloom drop. This is a common problem with container grown plants.

A tomato plant must be exposed to at least 6 hours of full sun every day. Less sun will result in poor blooming or bloom drop.

Additional Resource
http://edis.ifas.ufl.edu/VH028