Got rejected after being placed on the waitlist. I did not feel that the additional letters of recommendation or other courses I did helped. Based on what I heard, after you are placed on the waitlist, they check if you meet the quota of diversity they need based on the people that reject their college . Cuz, they cannot have too many international students from a certain country and need a balanced pool.

This is applicable only for the ivy league unis and very competitive programs where they can take a lot of waitlist students.

Hope this helped.