How does the 9^th Circuit decide whether a case is to be reviewed en banc—that is, by the full court?

For one thing, on the 9^th Circuit, en banc review isn't by the full court; there are too many judges (26 at the moment). So, their rules provide for "limited en banc" review, or review before an 11-judge panel. The court as a whole votes on whether the case is to be taken en banc, and this vote can be triggered either at the request of one of the parties or if one of the active judges wishes to have the whole court consider it.

The en banc court consists of 11 judges, of which the chief judge (that's Mary Schroeder at present) is always one. The others are selected at random, and it's possible for one or all of the judges from the original three-judge panel to be on the en banc panel as well. Another 9^th Circuit rule provides that if any one judge has not been on one of three successive en banc panels, that judge is automatically included on the next one. This ensures that unlucky judges are also represented.

Needless to say, this system can sometimes result in an en banc panel of 11 judges, of which a majority voted not to go en banc in the first place. Then they are stuck deciding a case they believed to have been correctly decided in the first instance. The court rules do provide for the possibility of the entire court rehearing the decision of the 11-judge panel, but if this has ever happened, I haven't heard about it.