From a physical standpoint, hiccups are caused by uncontrolled spasms of the diaphragm. Each spasm causes quick intake of breath, which makes a hiccup. The phrenic nerve, which controls the diaphragm, READ MORE http://www.wisegeek.com/what-causes-hiccups.htm
Most cases occur due to unknown causes.
At other times, a bout may be caused by spicy or hot food, carbonated drinks like soda, nervousness, intense emotions, sudden laughter, fumes, or cold air READ MORE http://www.digherbs.com/hiccups.html