What other reasons can an applicant is waitlisted? I know someone who got admitted EA to a selective college but is waitlisted on RA to a not so selective school?
They may be protecting their yield by admitting students who are more likely to attend.
Does the waitlisted school consider the applicant’s interest as a factor?
Don’t worry about other people and where they get in (you’re saying “I know someone” made me think you are not talking about yourself here). You don’t know any details of what is in the application and why he/she may be more appealing to one school than another (ex. parent alumni of a school, demonstrated interest in one school and not the other, has a particular interest/skill one school desires, did a great supplement for one school but rushed through the other… etc.). Just be concerned about your own applications.
Different schools prioritize different attributes. A smaller school might prefer applicants who can “double up”, as in play on a D3 team and play in the orchestra whereas a larger school will have enough students that neither of those things would matter.
One school may be eager for students who want to study xyz while that major may be oversubscribed.
You are trying to find the right school for you; schools are trying to find the right students. Don’t worry about it.
I’ve seen that a few colleges waitlist students when the the admissions committee vote ends up being a tie. They’re might not be any specific reasons for it, just that some were sure and some were unsure.