What causes a hiccup and why do people do it?

Answer 1

The diaphram muscles (a long muscle right under your breast bone) goes into spasms. Kind of like when your leg randomly starts twitching. It's not usually a medical concern, it's just irritating.

Answer 2

Simply put, hiccups are caused by the electrical signal sent by your brain to your diaphram (muscle for breathing), getting out of "synch." That is why a scare, or holding your breath, or anything that interrupts this autonomic electrical signal again, will often put it back in synchronization (thereby stopping the hiccups).