Hyperacusis is not well understood and has hardly been investigated as to its mechanism.

Some types of H appear to come from noise injuries,
but many other types can emerge from medications,
closed head-brain-neck injuries, poisonings or substance abuse,
and yet another type appears to be related to people who suffer from other inflammatory or chronic body illnesses, i.e.,
chronic fatigue, lupus, thyroid disorders, skin diseases, arthritis, etc etc so many possibilities...