What causes people to be midgets?

Genetics: Some persons have a spontaneous genetic mutation during conception of an infant, or a parent can carry and pass a gene to egg/sperm to cause dwarfism. We no longer call them midgets.

Dwarfism: Dwarfism has multiple causes, including genetic defects, endocrine dysfunction involving either the pituitary or the thyroid gland, chronic diseases such as rickets, renal failure, intestinal malabsorption defects, and psychosocial stress, as in the maternal deprivation syndrome. http://medical-dictionary.thefreedictionary.com/dwarfism.