There are 3 competing theories as to why he might be doing this now that come to mind.
1. He might be seeking to restore his badly damaged reputation, recognizing that the damage he has inflicted on the FBI is substantial. Three days ago, American Thinker published an open letter from a retired FBI Agent, Hugh Galyean, that laid out some of the damage Comey has inflicted on the institution he leads. There is little doubt that this reached many in the FBI family, putting in print what people have only whispered about. If those silenced voices start speaking out, Comey could face a serious loss of face. In this scenario, he is heading off a staff rebellion, possibly including mass resignations.
2. Rush Limbaugh today discussed an alternative theory, that by announcing an FBI Investigation resuming, Comey is putting a lid on further attention to Wikileaks. I guess this means that Clinton forces will argue we must wait for the investigation to be complete (after the election) before speaking about what the evil Russians are planting into our politics.
3. It is possible that something so dramatic came up in the pertinent emails that postponing a public reaction by not announcing the reopening of the investigation would, be regarded as political interference by covering up a smoking gun until after the election. In this scenario, Comey is assuming the evidence cannot be suppressed, and that he would be held accountable after it comes out. This scenario also indicates that we could be headed for a constitutional crisis, involving the possible indictment of a president-elect before an election. Or the evidence being turned over to the House of Representatives for impeachment hearings.
http://www.americanthinker.com/blog/2016/10/3_competing_theories_on_why_the_fbi_reopened_the_hillary_email_server_investigation.html4. All of the above.