The court’s decision appears to be sua sponte (on the court’s own initiative), as there is no record of any party to the case asking the court to hear the case en banc in the first instance. This has been done before, as with U.S. v. Microsoft in 2001. In that case, the court’s decision to go en banc could be explained by the fact that only seven judges could participate in the case, which could have made it difficult for the court to rehear the case en banc after a three-judge opinion. It’s possible that similar considerations were at play here, but the numbers don’t quite add up. Two D.C. Circuit judges (Supreme Court nominee Merrick Garland and Cornelia Pillard) did not participate in the decision to go en banc, but that still leaves nine judges — with plenty to vote for an en banc review of a wayward three-judge panel ruling.
The most likely explanation for the court’s order is that several of the judges (likely including one or more assigned to the original panel) concluded that some of the issues involved in the case, and perhaps the Clean Power Plan itself, are of such significance that they warranted the court’s attention in the first instance. While the immediate effect of the en banc order is to delay oral arguments by three months, the ultimate effect could be to accelerate review as it ensures that en banc review will occur sooner than it would have had the parties needed to wait for a three-judge panel before seeking such review. (This assumes that one or more losing parties would have petitioned for en banc review and that a majority of active participating judges would have found the case en banc worthy.) In the interim, the Environmental Protection Agency is precluded from taking action to implement its regulations, as they are subject to a Supreme Court stay.