The standard reason is too little water. Sorry if you've tried that.

Yeah, definitely not that! We've been getting rain almost every day lately.

Mine are doing round about the same thing. I thought that they might not have liked the Insecticidal soap I sprayed on the as they curled the next day.

And I did the opposite....sprayed Neem oil last night after I noticed the curling.

My cucumbers did that after I transplanted them. They eventually recovered. I thought it was lack of water too. I don't know what it was but I am blaming it on non-hardening. I am sure that is not your problem though. I assume you started them in the garden from seeds. There are so many knowledgeable gardeners on here. I am sure you will find your answer.