If a majority of the court's judges support en banc consideration, then the chief judge of the circuit, Judge Alex Kozinski, and 10 randomly selected appellate judges from the circuit will hear the en banc appeal, which can involve briefing and oral arguments.Why are they doing this? Probably because the very narrow opinion of the recent appeals court had a good chance of not being heard by the Supreme Court, which would have restored marriage to California. By doing this, they ensure that there is a further delay in the case, a further delay to the marriages, and another bite at the apple.
After that decision is reached, theoretically, a party dissatisfied with an en banc ruling of the Ninth Circuit can ask for the full Ninth Circuit to review the en banc panel's decision, but the court has not agreed to do so since adopting the "limited en banc" procedure.
After en banc consideration, the unsuccessful party could then petition the U.S. Supreme Court to hear the case. At that point, the parties submit written arguments explaining to the court why the justices should or should not hear the case. Then, if four of the nine justices agree to hear the case, another round of briefing occurs, with the parties and outside organizations and individuals arguing the merits of the case to the justices. Oral arguments are then set and held at the Supreme Court, and some time later a decision is handed down.
Also, I wonder if they (the pro-H8 side) are hoping for a broader finding than the recent opinion, one that would apply to other states, and would almost certainly be heard by SCOTUS and be struck down. They may be fishing for a bigger decision against them, in order to shut the door long-term. So even if they lose, they could still win.
There are a number of conservative, anti-gay judges in the 9th, so the outcome is not certain. Remember, a 9th circuit panel vacated the DADT case that the Log Cabin Republicans won. And one of the three judges on the recent appeal disagreed about Prop8.
It ain't over.