No one know for sure why people hiccup. A hiccup is caused by the diaphragm going into spasms that change our normal breathing pattern.
As you take in more air in the diaphragm, your mouth and throat are trying to keep air from entering. It becomes a battle between the diaphragm and the mouth and throat. The rush of air hits your vocal cords and makes a "hic" sound. When the diaphragm settles down after a few minutes, breathing becomes normal again and the hiccups are gone.