First of all, the investigation was not re-opened because it was never really closed.

Now go read Comey's letter. And don't miss the part where he says he has no idea whether the trove of emails on Weiner's laptop has anything to do with Hillary Clinton, but he's informing Congress that, maybe, they kinda, sorta, might, so he is letting Congress know just in case.

Again, he didn't even get the warrant to examine them until yesterday, two days after he wrote his letter.