Under the General Orders of the 9th Circuit Court of Appeals, any judge in the circuit may call sua sponte for an en banc rehearing of a case. What this means translated into ordinary language, any judge, active or senior, within the 9th Circuit, can make the call for a en banc rehearing on his own behalf without any request from either the plaintiff or defendant.
And this is just what has happened in the case of Peruta v. San Diego. The parties involved have been ordered to file briefs within 21 days saying why or why not this case should be given an en banc rehearing. Amicus briefs may also be submitted within this 21 days period.
Professor Eugene Volokh, while surprised that this happened with Peruta, notes the reasoning behind this call.
This sort of “sua sponte” call for rehearing en banc, even without a petition, is a pretty well-settled procedure, though not one that’s often used. Opinions of three-judge panels in the Ninth Circuit are binding on all future three-judge panels, and can only be reversed by en banc rehearing (or, of course, by the Supreme Court). As a result, even if the parties are content with the result of an opinion, and don’t seek rehearing, each judge has his own interest in the precedents set by his colleagues. Judges are therefore allowed to seek en banc rehearing (which will be granted if a majority of active judges agree to rehear the case), regardless of whether the parties file petitions for such rehearing.Given the strong Second Amendment rationale in the Peruta decision by Judge O'Scannlain, I'm guessing some of the more liberal judges on the 9th Circuit don't want to be bound by it. After all, the 9th Circuit has a reputation as the most liberal circuit. Only 18 out of the 38 current active judges - those not having senior status - were appointed by Republican presidents.
Voting on whether or not to grant an en banc rehearing will be by all of the active judges on the 9th Circuit. If a majority approves the rehearing then, under 9th Circuit rules, a panel consisting of the Chief Judge and 10 other active judges will sit en banc and rehear the case. The 10 judges are chosen at random from among the active judges on the Circuit. The Chief Judge as of December 1st is Judge Sidney Thomas - the dissenter in the Peruta case and a Bill Clinton appointee from Montana.
I should emphasize that the Peruta case hasn't gone to an en banc rehearing yet but that it could go to one if a majority of the judges call for it. We shall see.