What happens to the application materials sent/submitted with graduate school applications in US after admissions decisions are made?
At our school, we never keep written reasons for acceptance or rejection. These are made in a final faculty meeting with voice votes of yays and nays for each candidate that made it to the semi-finalist list.  Only the finalist list is recorded.

As to materials, they were shredded sometime after admissions season is over. It's a moot point now that everything is electronic.