The additional federal withholding is most likely coming from Form 8959.
If your wages are over $200,000 (Single, Head of Household), $250,000 (Married Filing Joint), or $125,000 (Married Filing Separate), you are subject to the Additional Medicare Tax. The amount of Additional Medicare Tax your Employer already withheld is calculated in part V of Form 8959. Line 24 of Form 8959 is added to your federal withholding amount on Line 16 of your Form 1040.
