From what I understand, the Glock 22 was an older weapon with a broken left rear rail. This somehow resulted in failure of the striker block. The weapon discharged when it was twisted in the holster. Unfortunately, I no longer have immediate access to the report the county did on the incident, but it was determined to be a weapon rather than human failure.
The department quickly replaced the Glock 22/23 pistols with Glock 17's and 19's.