most likely the IRS is claiming you underreported your taxable IRA distributions line 15b. Increasing line 15b can make more of your social security benefits taxable. It won't if your already being taxed on 85% of those benefits (line 20b is 85% of line 20a)
carefully check you TT entries for IRA distributions against the 1099-R's you got and make sure you have them all and that you enter them all correctly.
you may want to go to the IRS website click on get your tax record and review the transcript for the year in question. it should detail all the IRA distributions reported to them. It's always possible you never got one.
