I suspect this will inevitably be blamed on greedy capitalist building
  owners, but the crazy tax assessors have some blame here, too. They value
  these buildings in the hundreds of millions of dollars and asses taxes
  based on that valuation - and that was "ok" because the building owners
  could extract that kind of value from the buildings by raising the rents.
  Now they can't - but the valuation and the taxes owed stay the same. If
  they hadn't tried to squeeze every last tax dollar out of the real estate
  market in the first place, the buildings wouldn't have become so expensive
  to begin with.

Prop 13 ensures that most of these office buildings are not being taxed on the market value of the property. Prop 8 ensures that tax assessments go down when the market value of the property declines. Property tax is not the primary reason rent is so expensive in San Francisco.