In 2010, there were some high-profile incidents with passengers forced to sit in airplanes on the tarmac without food and water for up to ten hours. In one incident at Rochester, MN, an airplane landed at 1230 am and the airport staff refused to open the terminal to allow the passengers off the plane. To put it mildly, this was a stunning failure to provide even basic levels of humanitarian care. As a result of the six-hour delay, the airlines and airport operator were fined $175,000 by the Transport Department and rules were introduced which require airlines to allow passengers off the plane if it has been stuck on the ground for three hours.
As a result of this rule change, airlines responded positively with the number of runways delays reduced to a tiny fraction of flights, but with a slight increase in the number of cancellations. The Transport Department is not clear whether the increase in cancellation is due to the new rules. All that can be said with any confidence is that airlines are returning planes to the gates if there are delays. This is a trade-off. Passengers who stay on the plane are entitled to food and drinks, must be allowed to use toilets and, if necessary, given access to medical treatment. Assuming no safety issues, passengers must also be allowed off the plane after three hours even if on cheap air tickets. If the airlines default, the fines are up to $27,500 per passenger. Obviously this is a substantial penalty and the airlines have been anxious to avoid paying. Even so, some delays have been unavoidable. For example, a severe thunderstorm can hold flights on the ground as priority is given to getting incoming flights on the ground. These flights take the gates and leave the waiting planes on the tarmac.
In part, there's also a problem with a shortage of gates at some airports and a lack of people in the control tower. The issue is always whether returning an airplane to a gate will disrupt the operation of the airport. Since the fine falls disproportionately on the airlines, there's possible unfairness but, so far, passengers holding both full-price and cheap air tickets are winning.