Everybody's trying to figure out what's going on. The general feeling is that this was a long time coming because of the way Comey handled the Clinton email investigation:
Mr. Comey broke with longstanding tradition and policies by discussing the case and chastising the Democratic presidential nominee’s “careless” handling of classified information. Then, in the campaign’s final days, Mr. Comey announced that the F.B.I. was reopening the case, a move that earned him widespread criticism.Of course, the usual suspects are calling this a purge to throw off the Russia probe. As Charles Krauthammer noted, it took a Democrat less than an hour to make a "Saturday Night Massacre" reference.
Update - Assistant Attorney General Rod Rosenstein lays out the case.