I have always flown United from LHR to Dulles, and then on to family scattered over the US on the regional airlines that go to the smaller cities. Service on the smaller aircraft like the Embraer involved in this incident can be limited. Cabin crew can be overworked.
As others have posted, the system of overbooking ensures maximum use of the fleet. Calling for volunteers to step off for suitable compensation is still the best way to cope if everybody does show up who booked for that flight.
Apparently three seated pax had volunteered to take another flight, and the good doctor also apparently volunteered, but his wife was traveling with him and he changed his mind. When the cabin crew asked the computer then to choose by lottery a fourth pax to step off the plane, Dr. Dao's name came up.
I also wonder if the computer preferred to select a "volunteer" who had no baggage in the hold....