There's a bunch of causes. It's like suggesting there's one reason for road rage. The chances of sitting next to a disruptive passenger are probably less than on a Friday night in the pub. I get an incident every couple of weeks where it hasn't gone to violence, when the person has been nasty and horrible and the crew has talked and talked and resolved the situation. But if someone tries to get on the flight deck or decides they want to get off the plane at 35,000 feet, the crew has to be given the means to restrain them.
Most airlines carry plastic or metal handcuffs and other restraining devices such as straitjackets.
But try putting handcuffs on someone who has gone berserk. That requires training. International legislation to deal with disruptive passengers is based on the 1963 Tokyo Convention. In some countries, this has been enacted to allow an unruly passenger of whatever nationality to be arrested upon arrival with a foreign carrier. British law allows police to detain passengers on any aircraft arriving in Britain at the request of the captain.