One thing could be if you got any 1099R for IRA, 401k or pensions. It might have been reported on the wrong line 4 or 5 so the IRS missed it and added it on to your return again which made some of your SS taxable. If a 1099R has the little IRA box checked (between boxes 7&8) then it goes on line 4, otherwise no checkmark goes on line 5.
