The reason given is that Comey knew the material had been provided to the press, and was going to be published.
Comey says unequivocally that the Russians interfered with the election (and gain, the FBI made no effort to apprise ANY state actor, the people who run the elections, of ANYTHING.
That's because the interference was not unwelcome, or was of no concern.
Meaning, they expected tat any Russian involvement would benefit HILLARY, not Trump.