Certified GKH Gardening Expert
This could be due to a few things. Overfertilization with Nitrogen is a common cause. Overwatering is another. Calcium deficiencies can cause this, but usually this presents itself on fruit more than bloom. Issues with soil pH, and high temperatures will be another reason.
This article will help you to grow tomatoes to their potential:
This link will take you to a free ebook that you can download on the subject of tomatoes: