A bug in human software
This is my conclusion. Some years ago there was on ongoing mail discussion in one of the major medical journals (doesn't matter which one, since in the end it came to no great conclusions).
We all know about various circumstances that can trigger them, or at least it seems that way. We certainly see them arise in certain neurologic situations of posterior fossa or brainstem problems that by some mechanism get them going, and there are cases reported of hiccups lasting many years in an individual.
To me it can only make sense that they're a "bug" in the nervous system when you realize that they cannot have any possible physiologic purpose. Nothing good comes from them whatsoever. Mostly we can just be thankful that eventually there is something that resets itself to stop them.