What are the reasons (medical, emotional, etc) that a person shakes/trembles/shivers?

Many reasons. Hormonal-low thyroid makes you cold and shivering generates heat, low blood sugar causes adrenaline to be released; neurologic-pinched nerves cut connections betn brain and spinal releasing control of muscles, damage to parts of brain eg parkinsons; anxiety causing excess adrenaline, drugs like caffeine, stimulants mimic adrenaline.