There's a list of 'reasons' for return that the customer can pick from and click on. I think there's also an "other" on the list, so you might never know. I also don't think there's a way we (seller/author) can see what the customer selected for a reason.
I agree it would be nice to know the reason. I like to tell myself it was an accidental click to buy. It's on the list of reasons, so it's plausible. I've heard that some people on amazon prime click to buy instead of whatever the procedure is for getting a library loan. If you're enrolled in kindle select it's a possibility.
Still... it's only guesswork. Yup, sure would be nice to know. It might help us become better authors if we know what prompted a customer to ask for a refund.