Could it be that the conservative residents of California have slowly moved OUT of the state ?
U-HAUL ( the trucking and moving company ) has done yearly surveys of resident outflows and inflows and they tell us that hundreds of thousands of California residents have been leaving for the past 3 years and the net outflow EXCEEDS the inflow by hundreds of thousands.
Of course U-HAUL does not survey the political persuasions of those moving in and out but IF those moving out are the conservatives ( and there is good reason to believe that ),
it looks like most of those who choose to remain because of the generous welfare and goodies they get from Sacramento will be liberal.
If so, this explains the shift in general favorable attitude towards gay marriage.