First you need to identify what major you will be choosing, as the prompt is looking for how you decided to choose that major. This is speculation on my part, but it seems to me that CALS is looking for people who know what they want to do/study. I think CALS places an emphasis on practical application of knowledge, so people applying there should sort of already know what they want to do.
This being said I think your essay should demonstrate and convince the reader that the major you picked is a perfect fit and that you definitely want to study in that field. My essay talked about why I chose the major I did, and how I differentiated it from other ones to start out with. After that I just talked about some key life experiences which have kindled my passion for my major.
I know this doesn't have to do with the essay per se, but I think you should try to make your application tell a story if that makes any sense. I know you can't tailor your Common App to a particular school, but you should try to get aspects of your application to write a narrative of sorts. For example if you are interested in science: Heavy science courseload, teacher recs from science teacher, SAT II science one, essay w/life experiences about science/major, and maybe a EC or award that is science related. In this way you are leaving breadcrumbs for the adcom to follow.
*Little Disclaimer*, I am applying to Cornell ED too, so I'm probably not the most knowledgeable when it comes to applications. My post is just what I gathered from researching my major and reading college admissions articles.