I need some help understanding why I had $863 of additional taxes in 2019.

It is very difficult to know exactly where it came from without seeing the form.  If you have your 2019 tax return you should check it out.

The most likely is line 4 Self-Employment tax.  If you are self-employed there is an addition 15.3% tax for Social Security and Medicare.

Line 6 is a 10% tax on early withdrawal of retirement accounts.

For higher income earners there is:

Line 8a is an additional Medicare tax Form 8959.

Line 8b is a tax on Net Investment Income Tax Form 8960

Any of these and perhaps a few other less likely taxes may have contributed to the additional $863.