What can cause a blood clot in leg?

Immobility. Ie with a flight or long car ride, or immobilization . Could also be due to your genetics and a clotting problem.
Other diseases. Some diseases make a person hypercoagulable, so there blood clots more easily. One would need to worry about this kind of process as well as cancer which is linked to a hypercoagulable state.
Blood clot triad. Blood clots are caused by any or all elements of the triad of hypercoagulabiliy, venous stasis, and vascular injury. These were described by a German pathologist over 150 years ago and are still known as Virchow's triad for causes of venous thromboembolic disease. These can be patient related such as obesity, medical illness, etc or extrinsic such as major surgery, long haul air travel, etc.