It is possible like Meadowlark suggested that it leaked before, but was not noticeable. The drought just enhanced the the ability of the leak to become more by allowing the ground to dry and retain any moisture. That inability to maintain its integrity, IMO is the result of not holding water. I would look to see if an area(s) behind the dam that seem to be wetter than normal (in comparison to previous years).
The other thing i would look for as was suggested is for burrows from an animal. The water is moving, the hard part will be to determine where and why. It may bee a good idea to have a pond builder that know your ground and come look.