You had more income subject to taxes this year and or fewer deductions or exemptions. If you didn't pay taxes last year and none of those things changed then you wouldn't owe income tax this year. Tax rates did not increase in your tax bracket. Also you may not have had enough withheld from your paycheck this year. What I said refers to your Income Tax and not to the payroll tax for Soc security which increased for the 2012 tax year from the previous 4.2% to 6.2%.
Income changed, deductions/exemptions changed, you did the math wrong, etc. there has been no federal tax increase for people making less than $400,000.00 a year. Social Security returned to its normal 6.2% after the admin gave us a 2 year break of 4% but that's it.