One possibility is that you were subject to an IRS offset. This TurboTax FAQ explains what that is: https://ttlc.intuit.com/questions/2539782-what-is-a-refund-offset
Also, the IRS may have corrected a math error on your return.
In either case, you'll receive a letter of explanation.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.