Why am I seeing: You have Additional Medicare Tax (Form 8959), and that's just one of the reasons the IRS requires you to file Form 1040

It seems you may be liable for the additional medicare tax  because of your income level.  This additional tax is reported on Form 8959.

A 0.9% Additional Medicare Tax applies to Medicare wages, self-employment income, and railroad retirement (RRTA) compensation that exceed the following income threshold amounts based on filing status: $250,000 for married filing jointly; $125,000 for married filing separately; and.

When a portion of an individual's wages will be subject to the additional tax, but earnings from a particular employer do not exceed the $200,000 threshold for withholding of the tax by the employer, the employee is responsible for calculating and paying the additional 0.9% Medicare tax.