It’s hard to crack each individual college’s method for deciding which students get off the waitlist. Contact the admissions office or check the college’s website to find out if the college ranks waitlisted students or if it has a priority list. If you rank higher, you'll have a greater chance of getting off the waitlist.
But some colleges may neither rank waitlisted students nor accept them by priority. Individual colleges may take a more holistic approach. They may make their decision based on factors such as which majors they want represented.