Jeremy Stahl for Slate:

“It’s an extremely unusual thing for an entire office to take itself off of a case in the middle of a case,” one former DOJ attorney, who worked under this administration, told Slate. “[These are] attorneys with the highest standards of integrity and highest standards of professionalism. I can only imagine they were in the very difficult position of being forced to contradict factual representations they had made previously.”
“That the entire office is being substituted out really indicates […] something strange,” the former attorney, who requested anonymity due to concerns about professional repercussions, added.
Another former DOJ official also speculated to the New York Times that the unusual move was likely because the original legal team was refusing to reverse course.

“There is no reason they would be taken off that case unless they saw what was coming down the road and said, ‘I won’t sign my name to that,’ ” Justin Levitt, a former senior official in the Justice Department under President Barack Obama, told the New York Times on Sunday.
The Times further confirmed that not only would career officials from the DOJ’s federal programs branch—the branch normally responsible for defending administration policies in court—be leaving the case, but a completely new branch would be taking over.