If you sell RSUs immediately upon vesting, there is no capital gains, only ordinary income.
What likely happened is that the supplemental withholding rate (22% for supplemental income up to $1 million, and 37% for income exceeding that amount) was lower than your marginal tax bracket, and the tax you owed was a result of the supplemental income (the RSUs) incurring more tax liability than was withheld for them.
What likely happened is that the supplemental withholding rate (22% for supplemental income up to $1 million, and 37% for income exceeding that amount) was lower than your marginal tax bracket, and the tax you owed was a result of the supplemental income (the RSUs) incurring more tax liability than was withheld for them.