AA DFW-SFO became DFW-SJC before we even boarded. It actually worked really well for me as I live a few minutes away from SJC airport but for most passengers it was a total inconvenience. They offloaded everyone's luggage in SJC, arranged for two coach shuttles to take people to SFO.

No reason for this change was ever provided nor were any apologies offered. Very odd.