I don't think there's any designated rivalries now...all of the teams in the South play each other every year, and the LA schools trade off playing the Oregon and the Washington schools (UCLA hasn't played Oregon in the regular season last season or this one).
It was the earliest in the season that Cal and Stanford have played each other,
this is what Yahoo had to say:
The Cal-Stanford game was moved to the middle of the season to accommodate the Pac-12 television schedule. Officials for both Cal and Stanford opposed having the game played at midseason, but officials from the other conference schools approved the schedule. Not since 1896 has Cal faced Stanford in a game placed in the middle of the season. Having the Pac-12 title game set for the first weekend of December limited the options. The Cal-Stanford game could have been played the Saturday after Thanksgiving, but the two schools did not want the holiday to conflict with the events leading up to the Big Game. The game could have been played the Saturday before Thanksgiving, but officials from other conference schools vetoed that because of a variety of scheduling issues. The Cal-Stanford game will be played late in the season again in most future seasons.
