In addition, many people have difficulty being stuffed in close proximity with others and forced to share space, whether it's overhead bin space, armrests or space taken away when the person in front reclines. As planes get more crowded, people get more cramped. The decline in air service likely has also exacerbated the problem. Some people are simply mad at airlines and seek retribution.

"Putting a melting chocolate bar in a seatback is a way of punishing the airline for shoddy treatment," said Dr. Robert Bor, a clinical aviation psychologist in London.

What's more, air travel is largely anonymous and, removed from everyday reality, people perceive few consequences to bad behavior. Just the opposite is true: Federal law requires that travelers comply with crew instructions, and penalties for disruptive behavior aboard airplanes can be harsh.

Adding to the dehumanizing of travel is the advent of kiosks, self-service check-in and other money-saving automations. With little interaction with airline employees, some of whom may be grumpy and surly anyway, customers have little emotional connection to airlines, so why worry about trashing their airplanes?

"Some rather enjoy offending others," Bor said.