FBI director James Comey did something highly unusual and highly unorthodox when he announced, apparently without the consent or agreement of the agents working the Clinton email investigation, that the FBI would not be recommending prosecution for her security breaches. He then did something else quite unorthodox when he released to Congress, but under very tight restrictions, some of the documentation about the investigation. And now he has done yet another unorthodox thing in writing to Congress, 10 days before the election, about reopening the email investigation with the newly-discovered Clinton emails on top Clinton aide Huma Abedin's laptop.
A number of sources report severe internal dissension within the FBI about Comey's actions, including a number of pending high-profile resignations (insiders say he has a stack of resignation letters on his desk by now). And of course by now he has infuriated both the Republicans and the Democrats. Moreover recent revelations make it pretty clear that the investigation itself was highly unorthodox (unnecessary immunity deals, no grand jury empaneled to allow the FBI to issue subpoenas, agreement to destroy the relevant laptops, agreement to limit questioning, etc, etc). It certainly looks from the outside like the whole thing was fixed from the beginning to assure Hillary wouldn't face charges. And in fact the agent originally in charge of the investigation, John Giacalone, resigned in disgust midway through the investigation, telling friends the investigation was "going sideways" (Washington slang for being politically manipulated).
Some writers are guessing that Comey jumped at the chance to reopen the investigation in order to try to repair his reputation, and/or perhaps to repair the reputation of the FBI, and get some of the heat off of himself. We will see. My guess is that we won't see the results of this new investigation until after the election, and that no one will have the guts to charge a president-elect with a criminal case. Of course if Trump defies the odds yet again and wins the election, Hillary will indeed be in a world of hurt.