The TSA failed some in-house tests. When a student receives a10% on an exam, it's usually time to drop the class. When the TSA gets a similar score (missing 67 out of 70 bombs), it's "out of context"; an indication of weaknesses to work on; and time for a high-level scapegoat to be fed to the media.
(Melissa Block of NPR interviewed John Pistole, former TSA director, about this. It was completely mundane. My cynical side says she isn't a journalist, but rather just part of the media machine that can get interviews with officials anytime she wants as long as she never really presses then to actually inform taxpayers about what is really going on and how they are culpable.)