You didn't mention how long you've had the plants in the ground. Are they transplants from starts? Each year right after planting the squash transplants into the garden, they get a spurt of BER. To date, it has passed without our doing anything to modify or change things and then we get more yellow squash than we know what to do with. I think its because the plants get stressed when they get transplanted and once they adjust they are fine.
I've not tried the things Catgrass mentions but I've heard they can be helpful.