It is a particular risk for people who are not able to move around normally.
Blood flows slowly in the legs, allowing it to form clots.
If that clot travels into the heart or lungs it can get wedged a vein. That can lead to sudden death. Deep vein thrombosis has been dubbed the 'economy class syndrome' as it is frequently caused by long periods spent in cramped conditions. On a long flight, lack of circulation means that blood can suddenly change from a liquid to a solid state and produce a clot. Blood clots can also be caused by dehydration.