Lack of airflow. I once had several raised beds full of strawberries. They be loaded with berries every year, just start ripening, and then we'd get a few days of rain, and they'd all mold. In that case, it was the location... tucked on the southeast side of a barn, there just wasn't much airflow around them.
I replanted out in the open, and unless we got a monsoon week, didn't get any mold.
I don't know if there's anything that can be done at this point. Possibly snipping out some of the leaves to let more air in, or even setting up a fan? It spreads fast, sadly.
Summerthyme