RSUs are treated as supplemental income by the IRS, which is subject to a flat 22% withholding if you earn under $1m/year (37% if you earn over).

Most people who earn RSUs have a total income that's well above the 22% tax bracket, but earn less than $1m/year, creating a shortage in what is withheld vs. owed in taxes.

Check out the full video linked here for everything you need to know about RSUs and taxes.