Were you within the time frame of your mortgage contingency clause? They can ask for proof of denial if you are looking to get your down payment back. If you have exceeded the time frame for your mortgage contingency, or you told them you were approved, only to have your loan later denied, you would want to do whatever they ask to get your down payment back, because they don't have to give it to you.
Remember, you signed a contract, which is legally binding, and you will have to abide by the terms. Although it will not say whether you have to give them a reason why you were denied, they could ask, as you are the one breaking the contract.