How is my state refund significantly lower than what Turbo Tax quoted. I don’t owe any child support, back taxes etc.

One reason why you didn't get the $8,000 state refund may be because you entered the wrong State Income Tax Withheld amount when entering your W-2 Box 17. 

• If that's the case, there's no way for TurboTax to catch that error. TurboTax relies on the user to enter the correct amount so TurboTax can correctly calculate the refund or amount due. 
• In the future, if you're able to import your W-2, this error could be avoided.

the state will send a letter of explanation. but such a large refund would seem to indicate you made some sort of error when entering the data. 

The state sent a letter and I found my error right away. I’m glad they caught it! Now I know what to not do next year!

I'm glad your issue got straightened out and so quickly.

Input errors are very common and often create confusing and frustrating  issues with respect to your anticipated refund.

Click here for common reasons why your refund is not as expected.