Motive, Means, Opportunity.
Who would have the most to gain from corrupting Wikileaks documents except those whom it targets (and who have suffered huge embarrassment and loss of credibility because of such revelations)?
I’d say Uncle Sam could likely be the one to sprinkle bad stuff into Wikileaks files. Or if you want to be really paranoid then someone intent on discrediting Uncle by making it seem as though he was the one adding poison to the mix.
(Hello there, China.)