The problem is that California has a progressive income tax. It generates about one-third of total state revenue. Most Californians pay nothing, salaried workers pay a few percent of their income and a tiny proportion of high income earners pay nearly all the income tax (about 80%) at rates between 6 and 10%. Income taxes include capital gains and stock options.
As a consequence of progressive income taxes, the tax revenue to the state is highly volatile. In the most recent economic downturn, the tax revenues to California fell much faster than the economy. High income people have incomes that are more volatile than salaried workers. The effect is visible in State tax revenues.
Because the State saw falling tax revenues it was forced to cut, pair and shorten the hours of many union workers on the State payroll. Unions and their political voice, the Democratic Party, are, under the circumstances, willing to consider a trade off: greater stability of State tax revenues and government union jobs, for a more level income tax scheme.
Intelligence formed the tax investigation committee, will there still be intelligence left when the time comes to write and vote on a bill?