Why is there an estimated tax penalty if I my withholding for 2016 was more than my tax liability in 2015?

An estimated tax penalty appeared on my return, apparently because I owe quite a lot.  However, my withholding for 2016 was much more than my tax liability in 2015.  Where does the program ask for my 2015 tax liability so it knows that I withheld at least 100% of 2015 taxes?