According to Pro Football Talk's rumor mill, the Arizona Cardinals hand picked the 49ers for the home opener in their brand new stadium.
From Pro Football Talk:
A league source tells us that the NFL threw the Arizona Cardinals a bone by asking the team to identify the opponent against which they'd most like to christen the Cards' new digs.
And the Cardinals specifically selected the San Francisco 49ers.
Pro Football Talk goes on to assume that the pick of the 49ers was likely because the Cardinals felt that the 49ers were the weakest team on their schedule.