Not very many. So, the difference seems to be that others find reasons to get their work done and seniors do not. That means that seniors, who should best recognize among students the value of studying, suddenly stop. But they don't. Therein lies the cause of senioritis.
Seniors suddenly question the value of studying.
I could suggest any number of explanations. Considering the number of seniors that enter law school, medical school, and other graduate programs in the fall semester after they graduate and the success they enjoy, it is obvious that they haven't outgrown homework. This explanation is also suspect. These people might have more desire to blow off homework to spend the evening drinking at Tommy's with their friends. This seems likely.