My guess would be that you received Social Security benefits in 2024 and the increase in your AGI with the addition of your pension income has caused an increase in the amount of taxable Social Security benefits, increasing the marginal tax rate beyond the tax-bracket rate. That's not uncommon. Compare line 6b of Form 1040 with and without the addition of the pension income.
Other side effects of the increase in AGI are also possible, such as a limitation on some tax credit that depends on AGI.
