Robert MacLean became a Federal Air Marshal in 2001. In 2003, the Marshals received word of a potential hijacking plot. Unsurprisingly enough, because the Transportation Security Administration (TSA) fails at everything, they botched the response. Because of a budget shortfall, they cancelled all missions on certain flights.
Terror/hijacking alert? Meh.
MacLean discussed his concerns through the proper channels, but nothing was done. So he did what he thought was best: leak the story to the press.