Here is the detailed version of why and how things happened on this United flight http://www.mercurynews.com/2017/04/2...ntary-bumping/
"Summary of What Happened on United Express Flight 3411
Sunday, April 9, 2017
United Express Flight 3411 is regularly scheduled to fly Sunday through Friday from O’Hare to Louisville, with a planned departure of 5:40 p.m. CDT and an arrival of 8:02 p.m. EDT. Seating capacity is 70 customers.
Before boarding, flight 3411 was overbooked by one customer. Despite early attempts by United, via website/kiosk and multiple announcements at the gate asking for customers willing to take later flights, there were no volunteers. As a result, one customer who had not yet been given a seat assignment was involuntarily denied boarding (see Involuntary Denied Boarding Selection Process sidebar). The customer received a check as compensation and was booked on another United flight. The other customers were then called to board the plane.
At the same time, an earlier flight to Louisville, originally scheduled to depart O’Hare at 2:55 p.m. CDT was experiencing a maintenance issue (it was unclear if this issue could be fixed, but regardless, it would depart after flight 3411). Booked on this flight were four crew members, scheduled to operate the early Monday morning United Express flight from Louisville to Newark. Without this crew’s timely arrival in Louisville, there was the prospect of disrupting more than 100 United customers by canceling at least one flight on Monday and likely more. With this in mind, the four crew members were booked on flight 3411, creating the need to identify four customers who would not be able to take the flight.
United agents began to seek four volunteers, this time while customers were seated on the aircraft. The agent offered an $800 travel credit plus the cost of meals and hotel accommodations for the evening, but no customers were willing to accept the offer. The agent then followed the involuntary denial of boarding selection process to determine which customers would be asked to leave the airplane.
Once the four customers on flight 3411 were identified, the United supervisor spoke with two of the customers, a couple, who then departed the aircraft and received compensation.
The next customers approached were Dr. Dao and his wife. The supervisor apologized and explained they would also need to depart the aircraft, but Dr. Dao refused. The supervisor was unable to convince Dr. Dao to depart the aircraft. Given Dr. Dao’s unwillingness to deplane, the supervisor left the plane and spoke to the United zone controller, who indicated that authorities would be contacted. The supervisor went back on the plane to request again that Dr. Dao deplane and advised him that authorities would be contacted. At this point, one customer onboard the aircraft volunteered to change flights for $1,000 but United needed two volunteers in order to avoid having to remove the Daos. No other customers would volunteer unless United could guarantee an arrival in Louisville later that night. Given the fact that the 2:55 p.m. CDT departure remained on a maintenance delay, with a possibility of cancelling, United could not make that commitment.
Officers from the Chicago Department of Aviation, which has authority to respond to such airline requests and historically has been effective in getting customers to voluntarily comply, answered United’s request for assistance. These security officers were unable to gain Dr. Dao’s cooperation to depart the plane voluntarily.
At this time, the United supervisor left the aircraft and attempted to call a manager about the situation, and Dr. Dao, as evidenced by widely reported video footage, was physically removed from his seat by the Chicago Department of Aviation Officers. After being forcibly removed from the aircraft, Dr. Dao ran back onto the airplane and Chicago Department of Aviation Officers removed him for a second time. He was later taken to a local hospital.
All customers then deplaned. After approximately 40 minutes, the flight re boarded without the Daos and departed for Louisville."