When you add a substantial amount of income to your tax return, not only can you moved into a higher tax bracket, there are a number of other calculations that can change.
The most shocking for most taxpayers is taxable Social Security benefits. If you have Social Security benefits on your return, the extra tax is likely due to an increase in the amount of taxable Social Security.
For example, on a return where you have $30,000 in Social security income and $30, 000 in other taxable income, $13,850 of the Social Security income is taxable and the total tax is $3110.
If you add $14,328 to the above return, the taxable Social Security increases to $25,500 and the total tax is $7431. That makes it look like you are paying 30% on the extra income.
If this doesn't answer your question please respond back with more details about the type of income, deductions and credits you have on your tax return.
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