Certified GKH Gardening Expert
Unfortunately, there are many causes, and it can vary from plant to plant. Usually, if they are all experiencing this at the same time, then it can be due to there being too much nitrogen in the soil. This can be offset by adding potassium and phosphorus when it starts to flower.
Heat, and dry weather can also be the cause, but so can overly wet weather.
This article will help: https://www.gardeningknowhow.com/plant-problems/environmental/why-healthy-blooms-fall.htm