Nobody really knew why he did it... until he explained, in detail. Now we know. He thought Hillary was going to win, and after the Loretta Lynch meeting with B

Nobody really knew why he did it... until he explained, in detail. Now we know. He thought Hillary was going to win, and after the Loretta Lynch meeting with Bill Clinton, didn't want to seem like he favored Hillary to avoid backlash from the GOP and Trump's base. He needed to maintain his impartiality. And in that misjudgement, he did hand Trump the presidency.