Why do I owe additonal taxes to MD on state return?

Maryland has a sliding tax scale rate.  This means, if you had more than one employer, they may have withheld too little because each of your jobs would have been in the lower bracket.  It is also possible that your  employer simply miscalculated your withholdings and did not withhold enough for your wages.

If you think there is an error, you can also go back and double check your entries to be sure there isn't a typo in your W-2 entries for your state income or withholdings.   You will do this in the federal section under wages and income.   Then click edit to see your W-2 and verify your numbers making sure you have the decimal points and commas in the correct spot.