It began with United having a passenger removed due to overbooking that escalated into violent and continued through an American flight attendant having a hostile encounter with a passenger to Delta turning a flight around and removing a passenger for getting up and going to the bathroom while the flight was waiting to takeoff. These are separate incidents that should be considered for their own circumstances. United involved the forced removal and significant physical harm to passenger by security forces. American was a flight attendant meltdown with a passenger, that has happened before and probably will happen again given the stressful nature of flight attendant work including the emotional labor demands. Delta had a passenger violate FAA regulations and the company followed the rules for the violation. At heart, each event is different. What is common across the three is the outrage each has sparked among customers and potential customers. People who fly usually have reasons to dislike an airline. People have had flights cancelled, seats reassigned for no apparent reason, missed flights, had luggage delayed or lost etc. When perceived passenger mistreatment appears in the media, it becomes a catalyst for this residual anger and creates what can legitimately be termed public outrage. There is not simple crisis communication when a crisis produces outrage. Organizations still should try their best, United did not give its best while American did, but should not expect the outrage to simply evaporate. Crisis communication does not work miracles. In some crises, only time will allow the outrage to subside. However, crisis communicators should avoid poking the bear, that would be United, and adding fuel to the outrage. Give an appropriate response that shows concern for the victim. See Situational Crisis Communication Theory (SCCT) research for the evidence of the value of focusing on victims during a crisis.
I would like to add that of the three, only United is true crisis. The American and Delta situations are really paracrises, threats that are managed in public. American is high profile failure of customer service. The crisis response included effective customer relations resolution techniques. Delta was an FAA violation. We may not like the rules but their regulations that govern how people are the behave on airplanes. We do not smoke nor tamper with smoke detectors. Also, you do not get out of your seat on an active runway. Takeoff and landing are the most dangerous times during a flight. Yes, the man needed to go badly. However, if you break the rules (for whatever reason) you must be willing to face the consequences. Being on an airplane comes with legal constraints we all must abide for the safety of all. This is not a message that will resonate with outraged customers but it is the truth. Delta stood by its decision and that is unpopular but ultimately the right thing to do. Your frontline employees have to enforce rules and management needs to support them. The Delta incident was hyper-inflated by events on United and American to some degree. Timing can matter in a crisis and the timing of the bathroom incident was unfortunate for Delta because of the existing outrage.