https://www.ata.org/news/news/hyperacusis-related-damage-nerve-cells-inner-ear Research from John Hopkins University said most likely has something to do with a minority of outer hair cell nerve fibers acting as pain receptors.
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This page states hyperacusis is triggered by OHC's which release ATP which activates the Type II afferent nerve fibers to act as pain receptors. This happens naturally with 120db+ noise but the hypothesis is painful hyperacusis lowers the tolerance level that activates the Type II afferent nerve fibers once OHC's are damaged.