Why does my hand tremble sometimes?

Variety of reasons. There are many causes that could cause a hand to tremble - ranging from anxiety to essential tremor, overactive thyroid, or as a result of a side effect of a medication or a caffeinated beverage, for example. If you're particularly worried you should consult with your physician to rule out less common causes just in case, since I'm not sure about the circumstances during which your hand trembles.