Either you entered the wrong amount of their Social Security (SSA-1099Box 5)or the IRS changed the taxable amount of some other income like their 1099R. And by increasing their taxable income made more of their Social Security taxable.
The letter should say what they changed. A common thing is they increased the income for 1099R income. Either you missed entering one or you reported the wrong taxable amount on line 4b or 4d. Or it showed up on the wrong line 4b or 4d so the IRS missed it.
