Hmm, I'm afraid we don't have any additional light to shed on why those buyers are suddenly canceling via Google Checkout. It's possible that several Google Checkout accounts were compromised and used to place fraudulent orders but I'd recommend checking with Google about that. We've not heard anything.
If a payment processor tells us an order has been refunded, reversed or canceled we do disable the download links associated with that order at that time, but it would be true that the buyer had access to the file before canceling. It is an unfortunate cost of business in e-commerce. For more information on what you can do to dispute a payment that is being reversed by any given processor you will need to refer to their policies on reversals.