Comey did it by letter, not press release, and on a day when Dems traditionally dump bad news they can't keep quiet - the weekend news cycle.
This was coming out anyway, from NYC cops, WikiLeaks, and maybe his own troops. Team Hillary wanted to come out ahead of it and bottle it up as a "pending investigation". Media was supposed to ignore it and by Monday it would be forgotten. No actual "investigation" would be done, they'd just clamp a lid on everything for 11 days.
That was wishful thinking on Hillary's part. Media wouldn't fall on their swords for her, NYC wouldn't shut up, and Assange couldn't be stopped either.
This plan to stall until after Nov. 8th was a failure, but they had no other alternative.
Hmmm... Similar to the wishful thinking about the ME, which lead to 400,000 dead Syrians, for starters.
My possible take(s) on why Comey acted:
1) Comey said he would notify Congress if the status of the investigation changed, and he did. In case he was not decent enough to honor his word, if he had not notified Congress, and word got out B4 Nov. 8, he'd really be in deep doodoo, and the case for "draining the swamp", and him with it, would be all the much stronger in the eyes of average citizens.
2) The internal rebellion at the FBI could not be tamped down.
3) (Tin foil hat theory) If Comey had originally recommended an indictment, he would have been attacked as a Pro-Pub hack, DOJ / Lynch possibly could have put the recommendation "under review" and potentially (and very publicly) "investigated the investigation". There would be plenty of time before the election to, with the aid of the MSM, obfuscate and confuse the issue in the minds of average voters. Instead, and especially if he had a hint that more information might be coming out, Comey declines to recommend indictment, and Dems fall all over themselves to praise his integrity. Then, once he has enough new info. to reopen the investigation, he notifies Congress, and the Dems rush to attack Comey -- only now they expose their utter hypocrisy. (Obama sidesteps that last part.)
4) All of the above in various proportions.