I’m pretty sure U.S. law requires companies to withhold at 22% (or optionally higher) for any bonus/non-salary payments, which includes RSU vesting. Companies can choose to either “sell to cover” or just issue a proportionally lower amount of shares (e.g. you vested 1000 shares but only 780 show up in your brokerage account).

The problem occurs when 22% isn’t enough, which is often the case.