The roughly 34% marginal tax rate is likely due to either the additional $648 of taxable income increasing the taxable amount of some other income such as Social Security benefits, the pushing of some amount of long-term capital gains or qualified dividends to be taxed at 15% instead of 0%, or to a decrease in some sort of tax credit caused by the increase in AGI. You would have to examine your tax return side-by-side with and without the additional $648 to know what side-effect is the cause.
