A few different scenarios:

• Comey is a known Clinton hack going back to a soft sentence for Sandy Berger; he has financial ties to Clinton via a directorship at HSBC and campaign contributions to his wife, who ran for office as a Democrat-- in this scenario, this reversal was part of a dog and pony show and was intended to forsetall action by groups of FBI agents who are ready to "do something" on their own
• The Scott Adams scenario - Adams thinks Comey acted honorably in the first two decisions (first, to pass on indictment, and then to reopen the case); following Adam's logic, the third decision is understandable. I won't explain it myself, but you can find it here James Comey - As seen through the Persuasion Filter It is a good argument, but I don't think it's right.
• Comey is only trying to protect his job by responding to the most current political pressure. First, the democrats pressured him and he pssed on prosecution, then the resignations of a shitload of FBI agents pressured him again and he reopened, and finally he got more pressure from the dems, and flipped again.
• This last scenario is not going to happen as it is a forlorn hope of mine. The last decision that Clinton "wasn't guilty" is a smokescreen while the FBI gets in place to roll up the whole rotten group of hacks (her network of folks, and the folks at the Clinton Foundation, and every democrat hack at DOJ who has obstructed the investigation) -- not gonna happen, but I can dream