Seniority plays a part, but honestly, some of it is simply who is needed for customer contact. I work in a bank, and a branch employee has to be conscious of who in the branch is already out to take a vacation, because there simply won't be enough employees to handle branch traffic. A back office person, however, might have work pile up, but its not as urgent of a problem if something sits on their desk for a few extra days.
People who are higher up in the organization usually take time off.....RHIP.