A viral video of a man being dragged from his seat and forcibly removed from an overbooked United flight may have left many travelers with questions. Why, for example, do airlines overbook flights in the first place? And, what are your rights as a passenger?

A quick recap, a United Airlines flight to Louisville, originating at Chicago’s O’Hare International Airport on Sunday,was overbooked. According to a passenger who spoke with The Washington Post: An airline supervisor walked onto the plane and announced: “We have United employees that need to fly to Louisville tonight. … This flight’s not leaving until four people get off.”

The airline asked for volunteers to give up their seats. They didn’t get any takers and so, the airline picked four.

As passenger Tyler Bridges told The Post’s Avi Selk,  one man refused. And then the police showed up. Three of them. Several videos including one shot by Bridges, show the man, who said he was a doctor and had to get home to see patients the next day, pulled dragged out of his seat and through the aisle of the plane, screaming. Passengers are overheard saying, “This is horrible” and “What are you doing? This is wrong.”

But it wasn’t over. Bridges said about 10 minutes later, the man returned, clothes mussed and bleeding, “I have to go home. I have to go home.”

At that point, Bridges said, United officials cleared the plane. The next time Bridges saw the man, he was on a stretcher.

Here’s what United’s CEO Oscar Munoz had to say about the incident Monday via Twitter:

We’ve reached out to United with additional questions about how the airline decided which passengers would be bumped from the flight and what ultimately happened to the man in the video. We’ll let you know when we hear back. The Chicago Department of Aviation said late Monday that one of the officers involved had been suspended.

“The incident on United flight 3411 was not in accordance with our standard operating procedure and the actions of the aviation security officer are obviously not condoned by the Department,” the agency said in a statement. “That officer has been placed on leave effective today pending a thorough review of the situation.”

George Hobica, president of AirfareWatchDog.com, called the video “one of the most shocking airlines missteps,” he’s ever seen.

Instead of forcing passengers to leave, the airline would have been better off simply offering them more money to take another flight, he said.

“This make the leggings thing look like nothing,” he said. “That was a 1 in terms of a PR pr disaster. This is a 12.  I usually give the airlines a lot of leeway. It’s not an easy business, but this was just stupid.”

Hobica was referring to an incident earlier this month in which a traveler at Denver International Airport tweeted that two teenage girls were being barred from a United flight because they were wearing leggings. In the end, it turned out the girls were flying on company travel passes, which require travelers to follow the airline’s dress code. Still, some criticized the policy as dated and discriminatory against women. And, it became a public relations nightmare for the airline.

It’s not clear whether the passengers bumped from the Louisville flight recieved compensation, Horbica said. It all depends on the size of the scheduled flight. Those with 30 or fewer seats aren’t required to follow rules that govern bigger flights when it comes to being bumped, he said. It’s not clear how many passengers were aboard Sunday’s United flight.  It’s also not clear the flight can be considered overbooked in the traditional sense since passengers were being taken off to accommodate United employees, Hobica added.

Still overbooking is not uncommon and the incident has left many wondering about their rights.

Overbooking does happen, and according to guidelines set by the U.S. Department of Transportation it is not illegal. Most airlines, DOT notes, overbook scheduled flights to compensate for “no-shows.” In such a situation, DOT requires the airline to ask for volunteers. And those who do volunteer  — with a few exceptions — are entitled to compensation. According to DOT, it’s left to the airlines to determine the form or amount of compensation.

Here is what DOT says about passengers who are “involuntarily bumped:”

DOT requires each airline to give all passengers who are bumped involuntarily a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn’t. Those travelers who don’t get to fly are frequently entitled to “denied boarding compensation” in the form of a check or cash. The amount depends on the price of their ticket and the length of the delay:
If you are bumped involuntarily and the airline arranges substitute transportation that is scheduled to get you to your final destination (including later connections) within one hour of your original scheduled arrival time, there is no compensation.
If the airline arranges substitute transportation that is scheduled to arrive at your destination between one and two hours after your original arrival time (between one and four hours on international flights), the airline must pay you an amount equal to 200 percent of your one-way fare to your final destination that day, with a $675 maximum.
If the substitute transportation is scheduled to get you to your destination more than two hours later (four hours internationally), or if the airline does not make any substitute travel arrangements for you, the compensation doubles (400 percent of your one-way fare, $1,350 maximum).
If your ticket does not show a fare (for example, a frequent-flyer award ticket or a ticket issued by a consolidator), your denied boarding compensation is based on the lowest cash, check or credit card payment charged for a ticket in the same class of service (e.g., coach, first-class) on that flight.
You always get to keep your original ticket and use it on another flight. If you choose to make your own arrangements, you can request an “involuntary refund” for the ticket for the flight you were bumped from. The denied boarding compensation is essentially a payment for your inconvenience.
If you paid for optional services on your original flight (e.g., seat selection, checked baggage) and you did not receive those services on your substitute flight or were required to pay a second time, the airline that bumped you must refund those payments to you.

But how do airlines decide who gets bumped? According to the U.S. Department of Transportation, it is up to the airlines to set the criteria.

Airlines set their own “boarding priorities” — the order in which they will bump different categories of passengers in an oversale situation. When a flight is oversold and there are not enough volunteers, some airlines bump passengers with the lowest fares first. Others bump the last passengers to check in. Once you have purchased your ticket, the most effective way to reduce the risk of being bumped is to get to the airport early. For passengers in the same fare class the last passengers to check in are usually the first to be bumped, even if they have met the check-in deadline. Allow extra time; assume that the roads are backed up, the parking lot is full, and there is a long line at the check-in counter.

United outlines the criteria for bumping passengers in its “Contract of Carriage.”

Boarding Priorities – If a flight is Oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA. If there are not enough volunteers, other Passengers may be denied boarding involuntarily in accordance with UA’s boarding priority: