Why did this happen? Presumably because whatever conflict had required Judge Cornelia Pillard to recuse herself has been eliminated, but the court did not say.
One interesting consequence of this change is that there is now an even number of judges hearing the case, and this creates the possibility (however remote) of a 5-5 split. When the Supreme Court splits evenly, the judgment of the lower court is upheld without opinion (and without a precedential holding). The same thing happens if the D.C. Circuit splits evenly with regard to a trial court judgment.
Does it follow that a 5-5 vote is a win for the EPA? I would assume so — without a majority, the court lacks the power to vacate or remand the rule being challenged — but I do not know of a specific rule or precedent that speaks to this precise scenario.