From AA's contract of carriage:

"Involuntary denied boarding

"If there aren’t enough volunteers, we will choose customers to change flights involuntarily and deny boarding.

"Boarding priority is given to certain customers, including to those who:
Have special assistance needs
Are traveling as an unaccompanied minor
Have AAdvantage elite status
Paid for First, Business or Premium Economy
Checked in earliest

"The selection of customers who are involuntarily denied seats is solely at American’s discretion."