CA will be sending you a letter. They do not advise TT of any adjustments or offsets that they make.

I received a letter but it didn't explain anything.

The letter should contain a phone number that you can call for further information.  As @SweetieJean said, TurboTax would not have the information as to why CA reduced your refund.

Some possibilities:  You made a data entry error.  You forgot to claim some income. You owe back taxes, student loans, or child support.