As a guess?
1) Increased income of any kind could make more of a person's SS be subject to taxation, and the extra taxes are really on the SS income.
2) Not on SS yet? Some high income folks have an added Net Investment Tax (Form 8960).
3) The extra income actually pushed some of it into the next tax bracket, and you didn't realize how close you were?
4) Actually, it could also be a tax table issue. For the tax tables used, much of the table is listed in $50 increments, inside of which there is no tax change. IF your taxable income was close to the next $50 region, a simple (say) $1 addition of income would trigger (say) $12 in more taxes, while the next $49 added results in no tax change.
5) Something else? The extra income either reduced some other credit, or some other income deduction.
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The only way to really tell is to get a full copy of yoru tax return, both Before, and then After that CD interest was entered to see what else changed besides just the tax