Where do hiccups come from?

When you hiccup, your diaphragm involuntarily contracts. (The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdomen. It plays an extremely important role in breathing.) This contraction of the diaphragm then causes an immediate and brief closure of the vocal cords, which produces the characteristic sound of a hiccup.
What actually causes the hiccup is difficult to say -- in most instances, there is no obvious cause. Attacks of the hiccups seem to be associated with a few different things: eating or drinking too fast; being nervous or excited; and, having irritation in the stomach and/or throat. In some extremely rare cases, the underlying cause of hiccups can be pleurisy (inflammation of the membrane lining of the lungs and chest cavity), pneumonia, certain disorders of the stomach or esophagus, pancreatitis, alcoholism, and hepatitis. Any one of these conditions can cause irritation of the diaphragm or of the phrenic nerves that supply the diaphragm -- it's the irritation that causes the hiccups.

Still, the cause of most attacks of the hiccups remains a mystery. For the most part, hiccups are not significant or serious in any medical sense. Most bouts of the hiccups are brief, even though when you have the hiccups, they seem to last forever. In the rarest of all cases, an attack of the hiccups can last for a very long time (days) and lead to exhaustion. When this happens, it's possible that the person needs medical attention in order to "paralyze" the diaphragm and stop the hiccuping. Luckily, most cases of the hiccups can be "cured" by holding your breath!

when you swallow an air bubble

I think and this is off the top of my head from swallowing air while you eat.

I always get them when I eat sliced bread

Anything that temporarily weakens the muscle of the diaphragm. Muscle relaxers (including alcohol), abdominal distention from overeating, crying or even a number of rarer diseases that weaken diaphragmatic tone.

No one knows. One guess scientists have made is when you eat too much food, your stomach puts pressure on your diaphragm and irritates it, and it convulses.

your diaphragm muscle

I seem to get hiccups when I eat something really sweet, like those powdered sweettart straws, or cotton candy. My cure is to drink 7 or more gulps of cold water without breathing. Just holding my breath doesn't work, no matter how long I hold it.