Can you imagine how dead to rites Comey must have her to take this action.....
He had just been briefed. They just got the warrant to search the emails. They don't know what's there.
Comey, in my opinion, panicked and caved to Republican pressure. I think I read somewhere (Politico? I don't know, I'll take it with a grain of salt.) that there is a conservative group of investigators inside of the FBI that wanted Comey to push her harder, but the DOJ shut them down. I think he's been put in a really, really awkward position because he's on the wrong side of history no matter what he does. He's the perfect fall guy for both sides no matter what happens, and unless he does something, he's going to get crushed for action or inaction, internally and externally.
The likeliest thing is that they won't find anything (again), this won't impact the outcome of the election, and it will be nothing but fodder for Chaffetz to go on his holy crusade.
I also think that the more Clinton's team hammers the FBI, it will galvanize her base, which actually needed to happen. I think this may swing the polls 1-2 points in some states towards Trump, but that won't make a difference in her firewall. That won't be enough to flip NH, Colorado, the rust belt, PA, etc. Given early voting numbers in Nevada, she probably wins there, which is game over for Trump regardless.
I think this makes his loss more respectable, since he'll probably get Iowa and Ohio, and in an extreme case, Florida and NC, but his path to victory was always narrow to begin with, and this isn't going to give him the states he really needed.