When you amend your return TurboTax updates the Form 1040 in addition to creating Form 1040-X, even though the updated Form 1040 is not really part of the amended return. Compare the updated Form 1040 line by line to your saved copy of the original Form 1040 that you filed. That will show you which income items increased.
We can't see your tax return, but it is very common for additional income to make more of your Social Security taxable, so that could very likely be the cause of what you are seeing. Comparing the taxable Social Security amount on Form 1040 line 6b before and after adding the income will show you that.
If you need more help, it might be useful to know whether you are using TurboTax Online or the desktop TurboTax software.
