After you file

Is there an amount on 1040 line 6b or is it 0?

What usually happens is the IRS changed some other income on your return.  Like the taxable amount of a 1099R you got or you missed entering some income.  Like if a  1099R got entered on the wrong line 1040 line 4 or 5, the IRS would miss it and think you didn’t report it.

By increasing your other income it made more of your SS taxable.

Or There is a new question this year asking if you lived in a foreign place.  People have been answering it wrong or skipping it.  Go back through the Social Security entries.  

You can go back into your tax return but be very careful not to change anything.  Click on Add a State to let you back into your return.  You should see this screen after you enter your Social Security and hit Continue.  What is checked Yes or No?