This article by Hugo Martin explains:
The smartphone video that went viral last year showing a United Airlines passenger being dragged out of an overbooked flight prompted several large airlines to vow to end or dramatically reduce the number of passengers denied a seat. The nation's airlines have made good on that promise. *** or its part, United demonstrated last week how far it will go to avoid another embarrassing incident involving an oversold flight. United Airlines confirmed that it offered a passenger named Allison Preiss a $10,000 voucher toward future flights to give up her seat on an overbooked flight Thursday from Washington Dulles International Airport to Austin-Bergstrom International Airport.