The tax bill is a penalty for not making proper estimated tax payments. If you owe $1,000 or more in state and county tax that's not covered by withholding taxes, or if not enough tax was withheld, you need to be making estimated tax payments. If you did make estimated tax payments, either they were not paid on time or you did not pay enough to cover the tax owed.
For more information see Information Bulletin #3 or contact the Department of Revenue.