The increase shown by TurboTax is almost certainly accurate. You are likely experiencing side effects of the increase in your AGI. A major one is the multiplying effect on your tax-bracket rate due to more of your Social Security income becoming taxable. It's also possible that there is a reduction in some kind of tax credit. To discover the exact cause you'll need to compare your entire Form 1040 with and without the additional $20k of income.
