Two possibilities come to mind:

1) If you are getting the Earned Income Tax Credit (EITC).  The credit will increase as your income increases to a certain point, then will decrease as your income increases.  You may be in the first group.

2) If your income is in the $200-400k range and have high deduction amounts, you are likely getting hit with Alternative Minimum Tax (AMT).  Long topic, but in short, adding more deductions pushes more of your income into AMT.

Here's a quick AMT Summary