We can't really be sure what's happening, since we can't see your tax return, and there are many factors involved. Bur here are a few possibilities. The additional IRA distribution increases your AGI. That could reduce or eliminate various credits or deductions. You have to see what specific items on your tax return are changing when you add the distribution, besides the IRA distribution amount itself. The additional income could also make more of your Social Security benefits taxable. And it could be making you subject to Net Investment Income Tax (NIIT) or increasing the amount of NIIT if you are already there. So while the basic marginal rate applies to the IRA distribution itself, the added income has side effects that increase your total tax.
