Certified GKH Gardening Expert
Unfortunately, this could be for many reasons. To get a better idea of what it could be I will need description of the environment around the plants, and photos to see any damage. I can suggest some of the most common reasons, though. Unfortunately, it will require testing multiple things.
pH imbalances, either too high or too low, cause plants to die off. Too little or too much nutrients in soil can do it, as well. Too little or too much water, though it is usually too much that is more common, is another. Too little lighting is a cause. And diseased soil is common, as well.
You can test for these things, however. These articles will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-percolation-in-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/special/urban/contaminated-soil-city-gardens.htm