Since these kinds of leaks are very difficult to authenticate, there is an opportunity for intelligence agencies to game WikiLeaks by (slightly) altering stolen documents. I'd take everything in these documents with a huge grain of salt.
Or, the far simpler and more plausible explanation: We live in a world with a massive amount of networked, easily accessible data. Combine that with terrible security and controls, and leaving the final responsibility of data remaining "safe" to the end user (often times just an anonymous cog in the corporate machine) and we have the perfect mix of problems to result in non-public information being "leaked" to the ou…
Although much is made of the high-tech data mining services available to assist with discovery, having used some of them I sometimes wonder if it isn't cheaper and more effective to throw low-cost labour at the problem. Enter law students.