The most common causes of hyperacusis are

Hearing Loss, Noise Injury

Head Injury or Whiplash type injury

Acoustic Trauma

Adverse reaction to medication or surgeries

Chronic ear infections

Auto immune disorders

Unknown

Adults and children can develop hyperacusis: certain birth conditions are associated with hyperacusis, including Williams Syndrome and autism.
Since the auditory system connects the outer organs of hearing with the central nervous system, through a complex series of neural pathways, that literally pass through or coordinate with many diverse areas of the brain, there are endless possibilities for dysfunction that may contribute to hyperacusis.


http://www.hyperacusis.org/