Combination of things according to this:
James Comey's decision to revive the
investigation of Hillary Clinton's email server and her handling of
classified material came after he could no longer resist mounting
pressure by mutinous agents in the FBI, including some of his top
deputies, according to a source close to the embattled FBI director.
But Comey's decision to reopen the case was more than an effort to heal the wound he inflicted on the FBI.
He was also worried that after the presidential election, Republicans in Congress would mount a probe of how he had granted Hillary political favoritism.
So agents pissed at him have been causing enough pressure, and enough people resigning(which I've not heard anything about previously; 'threats to resign', yes, but not actual paperwork), combined with "This is making me look like crap!", added to fear of a real investigation into all this.
Hey, Comey, know what would've prevented this? Doing your damned job honestly in the first place.