It kind of grates on me when someone always answers in the affirmative with 'No, [affirmative]'. It has become more and more common (especially when I speak with people from California).

I think there's combination of forces at play. But I think a big factor is that people are afraid of appearing too agreeable. It would just be way to overt and enthusiastic to just say 'Yeah, totally' for Californian sensibilities.