As I recall it, one of their partner newspapers deliberately misunderstood how encryption worked, leaked their special passcode to one of the document archives, and then pointed at Wikileaks for being shitty after themselves handing the keys to the unredacted documents to the world.
The partner newspaper was in fact The Guardian.
The reason this was a critical screwup is that wikileaks reused decryption keys with data they shared so when one person leaked their key, it was usable on many people's encrypted data.