That and lackluster debate performances after the first. I think with Hillary there were a lot of people looking for an excuse to justify not voting for her because they just didn't like her or it was expected in their social circles that they shouldn't like her, so- having the FBI validating that 'there must have been something going on' played into that in big way IMO.
What frosted me is that Obama could deflected the whole episode onto himself and left it clear for Hillary by just saying that the White House knew what she was doing (which of course had to be true anyway) and decided to give her a pass on it. But there was some weird dynamic going on there where Obama was there helping grease the skids early to get HIllary the nomination, but then left her hanging out to dry when he could have helped her. Really have never understood it. Maybe just more of Obama's "I'm above politics" act, which he suffered from to his party's detriment.