The high admissions rate during SCEA is primarily driven by a better applicant pool, athletic recruits who almost always have to apply SCEA to get coach support and others with hooks, such as legacy and URM. Also, IMO, applicants who apply SCEA have Yale as their first choice and probably have done a better job with the various why Yale type of question (both in the nature of how Yale is going to benefit me and how I am going to benefit Yale). It is also suggested that if you have a very specialized skill (e.g. virtuoso french horn player), it is better to apply early to claim that “spot”. The Harvard litigation does suggest that applying early there is an advantage after controlling for a variety of factors…
Hooks are advantages that you have that you did not earn based on your efforts (with the exception of athletic recruit). The usual list are: athletic recruit, URM, legacy, child of major donor/political figure , faculty child, first gen, low SES.