It sounds like the Alternative Minimum Tax has kicked in for you. As the name says, it's an "alternative" tax rate which is calculated at the same time as your regular taxes. If the alternate method results in a higher tax, the difference between it and your regular tax is the AMT amount you'll have to pay.

AMT is extremely complicated, but the big offenders are State and local taxes, including property tax and personal exemptions (the number of people on your return).

Other things that affect AMT are long-term capital gains, tax-exempt interest, Incentive Stock Options, medical expenses, and miscellaneous itemized deductions (including employee expenses).

Mortgage (acquisition debt to purchase a house) bypasses AMT as do charitable contributions.