"So, you're the little lady who started this great brouhaha."
--Director Denton Voyles (The Pelican Brief)
Several stories (e.g., here, here) support the theory that we previously highlighted as to why FBI Director James Comey was willing to reopen the case against Hillary Clinton at this point in time. Simply put, he was facing an internal rebellion from FBI agents who were disgusted with the way the original investigation was handled.
If Comey did not formally reopen the investigation, then he knew that rogue agents would likely leak the new information, thereby implicating Comey as obstructing justice--perhaps for the second time. It seems not outside of the realm of possibility that he could even be arrested after having testified several times under oath that all evidence pertinent to the investigation had been considered in the case.
Add to that the likelihood of an outright meltdown of the Bureau if he didn't act, and Comey's motives for acting seem straightforward. He did it to avoid even more blowback than he is currently absorbing.