what causes turbulence on an airplane?

Changes in the flow of the air causes turbulence. Air is basically a liquid and it doesn't stay still all the time. Sometimes there is unpredictable movement in it. So when it moves around, it's fair to assume that it will move the plane around with it.

You can also make a google search about clear air turbulence, wake turbulence and wind shear.

Nothing causes turbulence  ON  an airplane like a drunken passenger OR a yapping pet ensconced under a passengers seat.

Turbulence in an airplane is a bit like rapids in a river. When the air around the airplane changes speed or direction, it pushes the airplane around a bit, and people on board feel this as turbulence.

The air up high is almost always moving. There's always a wind blowing in some direction, often a fairly strong wind. As an airplane flies along through this moving air, it is carried a bit by the air, just as a rowboat in a river is carried along by the current. As long as the wind is steady, and doesn't change speed or direction, people aboard the airplane don't feel anything. But if the airplane goes from one mass of air into another, and the two air masses are moving in different directions, or at different speeds, the airplane is jostled as it moves from one mass to another, and this is turbulence.

Winds can also be vertical, meaning that the air might be moving slightly upwards or downwards. Here again, if the wind changes, the airplane might be jostled up or down.

Usually the actual movement of the airplane is only a few feet. But since the airplane is moving at extremely high speed (600 mph in some cases), it moves very quickly from one air mass to another, and so it may be sharply bounced around as it makes the transition. It doesn't do any harm to the airplane, but it can be startling to people aboard who aren't used to it.

Airplanes can tolerate turbulence much better than passengers. Turbulence bad enough to give people motion sickness (which thankfully is quite rare) doesn't even faze the airplane. Turbulence that seems to be bouncing the airplane every which way is often actually so mild that the pilots can barely see it on their instruments.

There is a danger from turbulence, but not to the airplane. Passengers who are not strapped in with their seat belts can be tossed out of their seats if a sudden mass of heavy turbulence is encountered. In some cases, they can bang against parts of the cabin, causing broken arms, fractured skulls, and in rare cases even fatal injuries. That's why  the crew tells you to keep your seat belt on whenever you are in your seat. People who have their seat belts on are not hurt even in very heavy turbulence. Turbulence is the number-one cause of in-flight injuries, and the most common victims are flight attendants, because their job requires them to move around throughout the flight.

The only time turbulence is dangerous to the airplane itself is inside a thunderstorm. Inside thunderstorms, the turbulence is so severe that it can snap a plane into pieces. For this reason, airplanes never, ever fly through thunderstorms ? EVER.  Pilots will always steer well around a thunderstorm, and if a thunderstorm parks over an airport, departures and arrivals will be delayed or diverted until it moves away.