They were ground zero for a pair of US bubbles. This caused the state's revenues to skyrocket. Believing revenues would always skyrocket, groups which have essentially captured the state and municipality budgeting processes voted themselves large increases in benefits. It is politically impossible to reduce those benefits once enacted, yet they automatically snowball in costs every year. Its like the miracle of compound interest, except in reverse, and with the full principal being due every year.

The bubbles have burst and California no longer sees the potential for staggering revenue growth every year, but will see double-digit increases in benefits outlays for the politically powerful groups.

Bonus points: every time California has a short-term lack of funds they negotiate with their beneficiary groups to trade a lack of increase this year (note: not a cut, just less increase than they would have otherwise gotten) in favor of a new entitlement due in N years, whose NPV is astoundingly higher than the savings due to the lack-of-increase.