Clarifying some things here:
The ranking is from US News, which dramatically changed its formula for how it ranks colleges. Nothing about AU itself has changed, just the way that US News ranks. You can read their changes here: https://www.usnews.com/education/best-colleges/articles/how-us-news-calculated-the-rankings
One of the biggest changes is that they don’t take class size, terminal degree faculty, alumni giving, HS class standings, or graduate debt borrowing into account anymore, which were all categories which AU did well in (especially class size, which at AU averages 20 students per class).
You can read President Sylvia Burwell’s September message about the ranking here: Values, Choices, and the 2024 US News Rankings | Office of the President | American University, Washington, DC
Of AU’s sister/comparable schools, 19 moved down in rankings because of the methodology changes. US News has also been under extremely scrutiny for their rankings and methodology for many years, so while they are a factor in deciding, take them with an extremely large grain of salt.