This news came courtesy of the attorneys for each entity. We don't know when the subpoenas were actually issued; could have been today, could have been a month ago, who knows? But it is encouraging that the U.S. Attorney's Office is not publicizing this. Its job is to investigate, not to tarnish the reputation of (and possibly the jury pool for) the subjects of its investigations.
New Jersey Governor Christie stated at his now-legendary press conference two weeks ago that his office would "cooperate" with all "appropriate inquiries." There are two terms there which require definition: "cooperate" and "appropriate." The use of either term implies that actual cooperation without resistance may be a pipe dream.
Why were the subpoenas issued? This article quotes me with a partial explanation. The full explanation is that people who don't know about an investigation can destroy evidence and then claim they didn't know about it. When you issue a subpoena you are putting the recipient on notice that if they destroy anything they are risking being charged with obstruction of justice.
So what does this mean? I interpret it to mean the U.S. Attorney was worried about destruction of evidence. They're in a rush. This is serious.