Increase in the money supply and velocity of money. The money supply has increased dramatically and is why the bull market has been so long. However velocity is very low which has so far limited inflation. UBI would thus likely cause significant inflation.
The bay area is rich and the rents follow suit. They are still limited to the overall supply of money in the area. Obviously increasing supply of real estate would help the issue of course.
The total legal occupancy of SF is smaller than the number of people who want to live there. The city delays the issuance of permits for expanding occupancy.