There are many causes of this happening but the most common cause is overwatering especially if they are in pots. Have they been out in a lot of rain or in the rain and then watered?

They are planted in the ground. I use that old method of sticking a finger or two down to see how wet or dry the soil is. It rained overnight about 1/4 " so not sure it is too wet.

Take cuttings and put them in vases. They would grow roots quickly at this time if it's just root rot (overwatering). At least you will save some and start over.