The FBI reopened the case because they had no other choice

Comey's letter to Congress that the email investigation is back on was a surprise to everyone. There have been a lot of theories as to what's going on. Let me lay those out for you, and then describe what seems to be actually happening.

Everything is square.

 The investigation in July was done well, and they've somehow come across emails that are potentially relevant to the case. This is the stance of pretty much all Hillary supporters (but they'll assert the emails are not incriminating).

This is not a valid theory. We know for sure the investigation was poorly executed, and the reports from inside the agency (always anonymous) point to Comey obstruction, and to a workplace angry to the point of mutiny.

The theory fails because the first investigation was a sham. The Democrats pleading that surely the emails are irrelevant is wishful thinking. No way Comey, who covered for Clinton already, would open this up just before the election if it wasn't something catastrophic.

Comey played long ball.

I wrote in my blog post on Comey's congressional testimony that it was possible Comey was nixing the email investigation, because the real scandal was the Clinton Foundation. I urged for restraint for people not to slander the FBI when this was still possibly the case. However I've backed off the theory as revelations showed just how incredibly fraudulent the investigation was. It was one thing to let the fish off the hook, it's another to chuck the rod & reel out entirely.

The agents revolted

There is lots of talk that the agents in the FBI are quite upset. Whether they want to risk their careers and pensions over it is the question. And I have been pushing the notion that, until we hear of them taking action, they whole organization must be treated as corrupt and/or incompetent, and the members of it should bear the shame of being associated with it. The theory then is that the agents threatened they would go public if he did not indict her.

Blackmail

Just two days prior, on Hillary's birthday, there was some buzz that her 33 thousand deleted emails would be released. These were fueled by Twitter posts from Megaupload found Kim .Com and alt-right persona Mike Cernovich. Kim ended up posting a lead where, supposedly, the emails could be accessed by authorized people from the NSA.

Two days later the case was reopened. At first it was readily apparent that it was the missing emails. The notion was that was outside entity had access, and was blackmailing the FBI that, if they didn't move forward, the emails would be dumped to the public.

However, reports are that the emails came from notorious sex freak Anthony Weiner, wife of Huma Abedin, Clinton's top aid.

What really happened

A couple of months ago US Attorney Preet Bharara took up an investigation of the Clinton Foundation. This got some attention on Reddit. I had never heard of him, but many people had and said he was notoriously effective prosecutor who had put many corrupt bigwigs behind bars. They said if anyone could crack this open, it was him. I don't recall if anyone realized he was also investigating Anthony Weiner's sexual misconduct. Everyone pretty much forgot about this. It has been information overload with the leaks, hidden-camera confessions, and all that going on.

But here is the rub. Likely what has happened is in the course of his investigation, Bharara uncovered, at the least, classified information on both Abedin and Weiner's devices. I say at the least, because this doesn't even touch all the other scandalous information that may have been found.

Comey can't contain it anymore. This is outside their control. The only options were to continue to cover for Hillary and have them totally T-boned by Bharara, or to get ahead of the story to save face. Clearly they went with the second. 

What will happen

We don't know why Comey covered Clinton in such a high-profile case. Since July there have been many connection tied between the Clintons and Comey, and to his deputy director that oversaw the case. Whether Comey was angling to stay in office under Clinton, or there was other nepotism in place, or that he was being blackmailed (far more likely than you'd think, given how dirty everyone seems to be), we don't know if he'll continue to cover. But that is an option; he does everything possible to contain damage, protect her from the most severe charges, and keep the investigative eyes from the Clinton Foundation, which will reveal crimes perhaprs up to the level of treason against the state.

Or, seeing the tides turn, he may have to turn as well. And if he turns, he will have to follow Sun Tsu's advice and destroy his new enemy entirely. For his own good, he will have to see the Clinton machine completely destroyed. 

Obvious I hope for the latter. And if so, we'll see if Trump ends up forgiving him for his dereliction of duty in July.