LCDs can crack on their own. It's very rare but it does happen. If the outer glass is not cracked and does not have any markings on it, and the rest of the computer also does not have any markings on it, you may have a case you could make. You're probably facing an uphill battle though given how many people claim these screens cracked on their own, and the vast majority of these assertions are inaccurate. The cracking pattern itself does visually resemble a clear point of impact, which may make this harder to argue--that said, the outer glass not being cracked, chipped, or damaged may help your case.
I think the bottom line is that it is going to require some goodwill on behalf of Apple to replace this under warranty. Best I can say is approach it positively with a friendly demeanor, state your case, show appreciation for their consideration, and hope for the best. As much as we do criticize Apple on the forums, they often do show their customers goodwill and they might have a little goodwill in store here.