To be more accurate, the general brought down himself: McCrystal had developed an extremely inflated sense of self and irreplaceability, and was openly dismissive of civilian oversight (which is kind of worrisome coming from a man who directs weapons). The reporter simply reported how McCrystal was openly acting around him, leading to the administration taking action.

I only say this because people often talk about that as if the reporter had done deep black ops to gain covert information, thus making enemies throughout the establishment, when in reality he simply reported quotes directly fed to him, and in some ways acting on behalf of the establishment.