Many of my in-laws are from the East Bay, so I have something of a window into these issues. The 'southern' accent of the Valley towns is from their Okie migrant ancestors. As many fled the Dust Bowl, they carried their accents into the San Joaquin Valley and the Salinas Valley too. I've spent a bit of time out there in construction (SJ and Salinas Valleys) and most of the time those guys have little discernible accent. However, if you get them very angry or very drunk, it's nearly a southern accent. I've no idea where they get it from, as I've had a fair few glasses of wine at the holidays and can't recall hearing the southern brogue, but they all seem to slip into it when they get rear-ended on 5.