The part to blame is your diaphragm.
Next time you start hiccupping - take note! You can almost feel your diaphragm jerking and your vocal chords closing in your neck. I should know - I'm hiccupping right now!
So now you know HOW Hiccups occur, WHAT Causes Hiccups?
Sometimes eating too quickly or too much, an irritation in the stomach or the throat, or feeling nervous or excited can irritate the diaphragm. Simply put, anything that causes air to rush into the epiglottis in a fast or unpredictable fashion can cause hiccups. Almost all cases of the hiccups last only a few minutes. Some cases of the hiccups can last for days or weeks, but this is very unusual and it's usually a sign of another medical problem.
Weirdly, though, hiccups are as useless as they are annoying; they serve no apparent purpose. One hypothesis suggests they may be a remnant of a primitive sucking reflex.
Source(s): medicinenet.com, whatcauseshiccups.org, wikipedia