"It's a fishing expedition to see if Bannon can provide them with confirmatory evidence regarding the firing of [former F.B.I. Director James] Comey and obstruction of justice," said David Shapiro, a professor at John Jay College of Criminal Justice with FBI experience and a financial crimes specialization. "That might be the stronger charge here, rather than Russian collusion."

It isn't surprising that Mueller would want to interview a figure who had as much access as Bannon, experts say, even if only to corroborate the information the special counsel's team already gathered. Now, the question becomes: What is Mueller trying to get out of Bannon?

The court order, which arrived last week, was first reported by The New York Times on Tuesday.

FBI agents, at the time unaware that Bannon had retained lawyer William Burck the same day, arrived at his house in Washington, D.C., to serve him with the subpoena, people familiar with the events told NBC News . Burck is also representing two other witnesses involved in Mueller's probe into possible contacts between Donald Trump's presidential campaign and Russia.

Former White House chief strategist and Trump campaign chief Steve Bannon has reportedly been ordered to testify as part of special counsel Robert Mueller 's investigation — the first instance of a close Trump staffer or confidante being bound by a grand jury subpoena.

The use of a grand jury subpoena to bring Bannon in for questioning, however, did spur intrigue among some experts, breeding speculation.

"It's somewhat surprising" that Mueller would take this tack, said Jennifer Taub, author and professor of white collar crime law at Vermont Law School.

One explanation for the subpoena is that Bannon was refusing to cooperate with the special counsel voluntarily, requiring a more forceful invitation. "Maybe he just said no," Taub said.

Some sources told NBC that Mueller's move may have been to prevent the White House from stifling Bannon's cooperation. Indeed, Bannon on Tuesday told members of the House Intelligence Committee that the White House had instructed him not to answer questions about his role as an advisor to Trump. The committee subsequently issued a subpoena of their own to compel him to talk.

Another suggestion is that Bannon didn't want to appear as though he was running into the special counsel's arms to give his secrets away. With the subpoena over his head, he can credibly claim that his hands are tied and that he has no choice but to testify.

Yet Bannon received the subpoena at arguably the lowest point of his relationship with the president. Bannon had been gone from the White House for months by the time excerpts began to leak from Michael Wolff's tell-all book "Fire and Fury: Inside the Trump White House." Up until that point, Trump had been mostly complimentary when asked about his former advisor.

That facade crumbled soon after Bannon was alleged to called a Trump Tower meeting between Trump campaign officials and Russians "treasonous," among other statements. Bannon also insulted Trump's children Donald Jr. and Ivanka, according to the book. In turn, Trump attacked Bannon, both on Twitter and in a lengthy press release.

Trump tweet

Wolff's book might have played a part in the special counsel's decision, said Michael German, a former FBI special agent and current fellow with the Brennan Center for Justice's Liberty and National Security Program.

"It is interesting timing that the subpoena would come out after the book has gained so much attention, and that could certainly be part of it," German said. "That heightened sensitivity might have given Bannon more reason to say, 'hey, give me a subpoena.'"

German stressed, however, that there's no ruling out a coincidence.

"Having been involved in a ton of investigations, there are more coincidences than you can imagine," he said.

"That investigation is running on its own timeline that is completely opaque to the public."