For a not so serious answer to your question:
Tremor may be caused by:
Certain medicines
Brain, nerve, or movement disorders, including uncontrolled muscle movements (dystonia)
Brain tumor
Excessive alcohol consumption, alcoholism, or alcohol withdrawal
Multiple sclerosis
Muscle tiredness or weakness
Normal aging
Overactive thyroid
Parkinson's disease
Stress, anxiety, or fatigue (can cause a postural tremor)
Stroke
Too much coffee or other caffeinated drink